CLEAVAGE 241 



cleavage is frequently equal. When the protoplasm forms 

 merely a disc resting upon a large mass of yolk, it is obviously 

 impossible to speak of meridional or latitudinal cleavages; 

 hence the cleavages are described as vertical, either radial or 

 circular, and horizontal. The vertical cleavages soon become 

 connected below the surface of the ovum by horizontal planes, 

 separating the lower surface of the protoplasm from the 

 underlying yolk, and the peripheral circular cleavages similarly 

 separate the protoplasm from the outlying yolk (Figs. 116, 

 158, A). This is seen in the Teleosts, and in many Elasmo- 

 branchs, Reptiles, and Birds. After the protoplasmic blasto- 

 disc is divided into a number of cells, that is after it becomes a 

 blastoderm, other cleavages may occur parallel with the surface, 

 forming internal cells not visible upon the surface (Fig. 116), 

 and the blastoderm may thus come to be many cells in thick- 

 ness (Figs. 158, 105, D). 



Some interesting transitions are to be found between total 

 unequal cleavage and discoid cleavage, in those telolecithal 

 eggs where the accumulation of yolk is not as great as it is in 

 the Elasmobranchs, Teleosts, and some of the higher Craniates. 

 Thus in the ganoid, Amia, and some of the Urodeles, as well as 

 in the squid (Loligo), while cleavage is at first limited to the 

 upper or animal pole, the earlier cleavages gradually extend 

 down through the yolk mass and may finally divide it into a 

 few large cells. Here the more peripheral circular cleavages 

 (latitudinal) do not form any sharp separation between proto- 

 plasm and deutoplasm, and the yolk mass is for a long time 

 only partially divided by meridional cleavages alone (Fig. 113). 



Superficial. This type of cleavage is characteristic of Arthro- 

 pods in general and occurs elsewhere only in a few Coelenterates. 

 The central accumulation of the yolk, as it occurs in these forms, 

 is an unusual condition, and correlated with this we find 

 several unusual features in cleavage. Of course in such an 

 arrangement the most obvious distinction between animal 

 and vegetal poles is entirely lacking, and usually the position 

 of the polar bodies and the external form of the egg are the 

 only outward indications of the polarity of the ovum. Before 



