126 GENERAL CONCEPTS 



37. Embryonic Development 



The division, growth and differentiation of a fertihzed egg into the 

 remarkably complex and interdependent system of organs which is the 

 adult animal is certainly one of the most fascinating of all biologic 

 phenomena. Not only are the organs complicated, and reproduced in 

 each new individual with extreme fidelity of pattern, but many of these 

 organs begin to function while they are still developing. The human 

 heart begins to beat, for example, during the fourth week of gestation, 

 long before its development is completed. 



The early stages of development of practically all multicellular 

 animals are fundamentally similar; differences in development become 

 evident somewhat later. 



When fertilization has been accomplished, the zygote divides re- 

 peatedly by mitosis, forming a ball of smaller cells known as a blastula. 

 These early cell divisions by which a many-celled embryo is formed are 

 called cleavage. The pattern of cell division is determined largely by 

 the amount of yolk present in the egg. An isolecithal egg has a relatively 

 small amount of yolk distributed more or less evenly throughout the 

 cytoplasm. Telolecithal eggs have a large amount of yolk which is more 

 concentrated at the lower or vegetal pole of the egg; the active cyto- 

 plasm is concentrated at the upper or animal pole. The frog egg is 

 about half yolk and a bird egg is more than 95 per cent yolk; the 

 cytoplasm of the latter is restricted to a small disc at the animal pole. 

 The insect egg is an example of a centroleclthal one; the yolk accumu- 

 lates in the center of the egg and is surrounded by a thin layer of cyto- 

 plasm. 



The line of the first division in the cleavage of an isolecithal egg 

 passes through the animal and vegetal poles of the egg and forms two 

 equal cells, called blastomeres (Fig- 6.7). The second cleavage division 

 passes through animal and vegetal poles at right angles to the first and 

 divides the two cells into four. The third cleavage division is horizontal. 

 Its plane is at right angles to the planes of the first two divisions, and 

 the embryo is split into four cells above and four below this line of 

 cleavage. Further divisions result in embryos containing 16, 32, 64, 128 

 cells and so on until a hollow ball of cells, the blastula, results. The 

 wall of the blastula consists of a single layer of cells and the cavity in 

 the center of the sphere, filled with fluid, is called the blastocoele. Each 

 of the cells in the blastula is small, and the total mass of the blastula 

 is less than that of the original fertilized egg, for some of the stored 

 food was used up in the cleavage process. 



The single-layered blastula is soon converted into a double-layered 

 sphere, a gastrula, by the process of gastrulation. In isolecithal eggs, 

 gastrulation occurs by the pushing in (invagination) of a section of one 

 wall of the blastula (Fig. 6.8). This pushed-in wall eventually meets the 

 opposite wall and the original blastocoele is obliterated. The new 

 cavity of the gastrula is the archenteron (primitive gut), the rudiment of 

 the digestive system. The opening of the archenteron to the outside is 

 the blastopore, which marks the site of the invagination which pro- 



