144 



GENERAL TRINCirLES OF ZOOLOGY 



2. Unequal dcavagc. — The volk is ahundant, but not in such a quatitity 

 as to j>re\cnt complete cleavage; it lies especially at the vegetative pole of the 

 egg, causitig 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 i^ldolccilhal, holoblaslic eggs, figs. 103 and 104). 



Fig. 10^. — Unequal cleavage of the egg of Ptiroii:y:oii (after Shipley, from 

 Hatschek). .4, stage of eight cleavage spheres; B, blastula in meridional section. 

 The dissimilarity of the clea\'age splieres begins with the equatorial furrow. 



Fig. 104. — Unequal cleavage of a snail s egg, A'(i.\.sd miiljl>ilis (after Bolirelzky). 

 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. DiscoiJal 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 aiilage of the 

 embryo, or blasloderm (telolecithal, meroblastic eggs) (tigs. 102, 105). 



4. Siiperfieial 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. 



