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Developmental biology

Embryology is the science ,which deals with the development of the organism.
Development is the series of changes that take place in the zygote (fertilized egg or buds formed in asexual reproduction).
development can be divided into 2 phase:-

1 pre-natural or embryonic development

- this is the development which takes place with in the egg,the study of the changes taking place during this period is also called embryology.

2 Post natal embryonic development

- this is the period of development from the birth of the organism upto death of the organism.
- Ontogeny is the entire development of the fertilized. egg :-where as phylogeny is the ancestral or evolutionary history of species.

types of Eggs

- eggs of the animal of different kinds.on the basic of the amount of yolk present they are :-
1 Alecithal types in which the ovum contains almost no "yolk E marsupials "and "eutherian mammals".
2 Microlecithal egg contains the small amount of yolk .eg sea urichn.
3 mesolecithal types .egs contains the moderate amount of yolk .eg lungfish,frogs , toad etc.
4 Megalecithal or Macrolecitheal or Polylecithal eggs.

on the basis of the distribution of yolk eggs are-

1 homolecithal or iso lecitheal eggs in which yolk is eventually distributed .This is found in alecit eal and microleitheal eggs
2 telolecithal eggs are those in which most of the yolk lei near the vegetal pole , cytoplasm and nucleus lie near the animal pole such condition is found in mesoleithal and megalecithal types of eggs.
an in some cases as in insect centrally placed yolk is surrounded by thin cytoplasm .So such eggs are known as centrolecithal.
b In chick the huge amount of yolk occupies.The enitro ovum out cytoplasm and nucleus lie in the animal pole. This condition may be referred as highly telolecital .

patterns of cleavage

fig cleavage


a regular manner of furrows divide the egg in definite patter which is different amoung animal the patterns are:-

1 radial -

due to this the blastomeres are arranged radially around the axis passing through animal to vegetal pole. Eg spones , Echinoderms.

2 Bilateral o vegetal pole

in this case, the blastomeres of the right side are similar to those on the right side. EG molluscs ,vertebrates etc.

3 Spiral

in this case, blastomers are arranged in a spinal around the axis passing through animal the annelids.

types of cleavage

the cleavage is effected due to the yolk resulting in different types of segmentation or cleavage ,these are as followed
1 Holoblasticd
- this divide completes the blastomeres formed may be of equally in size.
- cleavage is holoblastic and equal in microlecithal types of eggs.
2 Meroblastic
- in this case, divisib occurs, only in the cytoplasm of macroleithal eggs , but not in the yolk.
- when they divide is restricted in the cytoplasim disc situ on the top of yolk it is called discoidal. It is found in repiles ,chicks etc.
- Meroblastic cleavage may also be superficial this occurs in the centrolecithal eggs of insects.
- in this case nucleus present in yolk divides many times and resuls nuclei migrate to the periphery in the cytoplasm.
3 Blastula
- after cleavage a solid balls stage is called morula is formed .it then chages into hollow ball stages the" blastula" which is one layer thick containing a cavity called blastocoels.
they are 4 types of blastula :-
a stereoblastula
such a blastula has large and few cells surrounding a small blastocoles or without a blastocoels. Eg hydra , annelid.
b Coeloblastula
Around a centrally or eccentrically placed balstocoel , one or more layers of cells are present .Eg star. Fisg , frogs etc.
c discoblastula
- in this balstula over the yolk are found few layers of cells forming a disc.
- the lower layer separates and forms a blastocoels surrounded by upper and the lower layer of cells. Eg fished , repitals and birds.

d Superficial blastula
- they have no blastococoel.
- the yolk surrounded by one layer of epithelial cells. Eg insect with centolecitheal eggs having superficial cleavage.


Agrawal, sarita. principle of biology. 2nd edition . kathmandu: Asmita book Publication, 2068 ,2069

Mehta, Krishna Ram. Principle of biology. 2nd edition. kathmandu: Asmita, 2068,2069.

Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.

  1.  Microlecithal egg contains small amount of yolk eg sea urichn 
  2. mesolecithal ypes egs contains moderate amount of yolk eg lungfishns,frogs , toad etc.
  3. Megalecithal or Macrolecitheal or Polylecithal eggs
  4. in this case the blastomers of right side are similar to those on the leght side . EG mollus ,vertebrates etc

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