144 



GENERAL PRINCIPLES OF ZOOLOGY 



2. Unequal Cleavage. The yolk is abundant, but not in such a quantity 

 as to prevent complete cleavage; it lies especially at the vegetative pole of the 

 egg, causing the cleavage in this region to progress more slowly; here larger 

 cleavage spheres are formed, because richer in yolk; hence the embryo, from the 

 first, is composed of smaller animal cells poor in yolk, and larger vegetative 

 cells rich in yolk (tehlecithal, holoblastic eggs, figs. 103 and 104). 



B 



FIG. 103. Unequal cleavage of the egg of Petromyzon (after Shipley, from 

 Hatschek). .1, stage of eight cleavage spheres; B, blastula in meridional section. 

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



FIG. 104. 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) . 



b. Meroblastic Eggs with Partial Cleavage. 



3. Discoidal Cleavage. The yolk is so abundant in the vegetative portion of 

 the egg that it prevents cleavage; cleavage, therefore, is limited to the region 

 around the animal pole and here forms a disc of small cells, the anlage of the 

 embryo, or blastoderm (telolecithal, meroblastic eggs) (figs. 102, 105). 



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

 prevents cleavage; consequently only the outer layer of the egg divides, forming 

 a superficial layer of cells, enclosing the unsegmented central mass (centrolecithal 

 eggs, fig. ^106). 



Distribution of the Types of Cleavage. Of the four types of cleavage 

 mentioned the superficial occurs exclusively in the arthropods. The others 

 are distributed as follows: the discoidal in the majority of the vertebrates and in 

 the most highly organized molluscs, the cuttlefishes and a few arthropods and 

 tunicates, while the equal and the unequal cleavage can be found in all groups of 

 Metazoa. 



