CLEAVAGE 



59 



Total cleavage 



Total cleavage is the rule in the small, non-yolky eggs of most marine 

 invertebrates. In the eggs of most species of echinoderms, molluscs, 

 coelenterates, worms, etc., only rather httle yolk can be distinguished. 

 The first cleavages in such eggs usually cut them into cells of roughly 

 equal size; such cleavage cells are often known as hlastomeres. In most cases 

 the equality between them is not very exact, and it usually does not last 

 through many cleavage divisions. Indeed, the most interesting aspect of 

 the cleavage of these eggs is the fact that there are characteristic patterns 

 of large and small cells into which the eggs of a particular group become 



Figure 4.1 



A. Lateral view of very unequal cleavage in the yolky egg of a sturgeon. 



B. Diagrammatic section of a late cleavage stage in the chick. The blasto- 

 derm is beginning to delaminate into two layers, the hypoblast (endoderm) 

 below and the epiblast (ectoderm and mesoderm) above. S.B.C. subblasto- 



dermic cavity. 



divided. In such patterns, there is often no obvious relation between the 

 size of the cell and the yolk content. The cleavage pattern must be de- 

 termined in some other way (cf p. 67). 



Examples of the main cleavage patterns are described in more detail in 

 the chapters dealing with the different groups. Here we shall merely list 

 them, for reference. 



(i) Radially symmetrical. Best seen in echinoderms. In these the first 

 two cleavages are vertical, the third horizontal, but a more complicated 

 pattern begins to appear at the fourth division, in which the cleavage 

 plane is vertical in the animal half and horizontal in the vegetative half, 

 where it cuts off a lower ring of quite small cells, known as micromeres. 



