GENERAL EMBRYOLOGY. 



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remarks a brief explanation will suffice to render intelligi- 

 ble the following figures of the various modes of cleavage. 

 a. Holoblastic Eggs with Total Cleavage. 



1. Equal Cleavage. The yolk, present only in small 

 quantity, is distributed equally through the egg; upon 

 cleaving, the egg divides into parts of approximately the 

 same size and equally rich in yolk (alecithal eggs) (Fig. 93). 



2. Unequal Cleavage. The yolk is abundant, but not 

 in such a quantity as to prevent complete cleavage ; it lies 

 especially at the vegetal pole of the egg, causing the cleavage 

 in this region to progress more slowly ; here larger cleav- 

 age spheres are formed, because richer in yolk; hence the 

 embryo, at the very first, is found to be composed of smaller 

 animal cells poor in yolk, and larger vegetal cells rich in 

 yolk (telolecithal, holoblastic eggs, Figs. 97 and 98). 



FIG. 97. Unequal cleavage of the egg of Petromyzon. (After Shipley, from Hatschek.) A, 

 stage of eight cleavage spheres ; B, blastula in meridional section. The dissimilarity of 

 the cleavage spheres begins with the equatorial furrow. 



FIG. g8. Unequal cleavage of a snail's egg, Nassa mutabilis. (After Bobretzky.) I, the 

 first meridional furrow has divided the egg into unequal parts ; II, the second meridional 

 furrow has formed three smaller and one larger cleavage sphere (seen from the side) ; 

 III, the equatorial furrow has formed four smaller animal and four larger but unequal 

 vegetal cells (seen from the animal polei. 



b. Meroblastic Eggs with Partial Cleavage. 

 3. Discoidal Cleavage. The yolk is so collected in the 

 vegetal portion of the egg that it prevents cleavage ; 



around the 



cleavage, therefore, is limited to the 



region 



