154 



GENERAL PRINCIPLES OF ZOOLOGY. 

 A B 



Fio. 100. Unequal cleavage of the egg of Petromyzon. (After Shipley, from Hat- 

 schek.) A, stage of eight cleavage spheres ; JB, blastula in meridional section. 

 The dissimilarity of the cleavage spheres begins with the equatorial furrow. 



in 



. 101. 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 vegetative cells (seen from the animal pole). 



cells are in straight lines (fig. 

 100, A), but in others the cells 

 alternate (fig. 102) ; this is called 

 spiral cleavage and is common in 

 several groups. 



b. Meroblastic Eggs with Partial 

 Cleavage. 



3. Discoidal Cleavage. The 

 yolk is so collected in the vegeta- 

 tive portion of the egg that it 

 prevents cleavage; cleavage, there- 

 in crepiduia. fore, is limited to the region 

 around the animal pole and here 

 forms a disc of small cells, the anlage of the embryo, or blasto- 

 derm (telolecithal, meroblastic eggs) (figs. 99, 103). 



4. Superficial Cleavage. The yolk is collected in the centre of 

 the egg and prevents cleavage; in consequence of this only the 

 outer layer of the egg divides into cells, which, in the form of a 

 continuous superficial layer, enclose the unsegmented central mass 

 (centrolecithal eggs) (fig. 104). 



FIG. 102. Spiral cleavage i 

 (After Conklin.) 



