806 TEXTBOOK OF ZOOLOGY 



to permit a clear-cut designation of the types of cleavage. Bilateral 

 cleavage is greatly influenced by the amount of yolk, and irregu- 

 larities in the cleavage pattern develop much earlier than in the radial 

 type. The degree of organization present in the egg at the beginning 

 indicates earlier differentiation, both physiological and morpholog- 

 ical, and embryos of this type are correspondingly reduced in their 

 power of regulatory adjustment. 



In spiral (Fig. 415) cleavage the tendency toward earlier organiza- 

 tion of the egg is carried to a further extent, and consequently these 

 eggs are among the most extreme of the types spoken of as determina- 

 tive; that is, they have a very much reduced power of regulation and a 

 much more precise sequence of stages leading to the formation of the 

 embryonic body. This type of cleavage occurs in the annelids and 

 mollusks, and in some smaller groups, for example, the subclass, Poly- 

 cladida, of the flatworms. 



Eggs of the meroblastic type contain so much yolk that their cleav- 

 age is quite modified from the more simple ones, previously men- 

 tioned, of the holoblastic type. There are two general kinds, discoidal 

 and superficial. In the discoidal eggs (Fig. 416) the yolk is in the 

 form of a single, fairly large and fairly dense body, surrounded l)y a 

 very thin cytoplasmic film which is totally unable to divide the entire 

 mass. It is possible to derive this type of cleavage from the extremely 

 telolecithal eggs of the bilateral sort by considering the amount of yolk 

 to increase and concentrate and the amount of cytoplasm to decrease 

 in proportion to the egg. Upon fertilization the cytoplasm accu- 

 mulates on one side of the egg to form a small disc-shaped or lens- 

 shaped mass. It is only this discoidal portion of the cytoplasm that 

 undergoes cleavage; hence the term applied to it. It divides suc- 

 cessively into two, four, eight cells, all lying in one plane, but at the 

 fourth or fifth cleavage some cells are cut off interiorly and there 

 arises a disc-shaped mass, several cells in thickness. With continued 

 cell division the discohlastula is produced. After gastrulation and 

 the formation of the germ layers, this cap of cells grows down, so 

 that it entirely encloses the yolk mass ; but at no time does the yolk 

 undergo any sort of division. Discoidal cleavage occurs chiefly in 

 the vertebrate classes, Elasmobranchii, Teleostei, Reptilia, and Aves. 

 There is also a type of discoidal cleavage to be found in the Cephalo- 

 poda, which is an extreme case of the spiral type, and a further 



