INTRODUCTION 



27 



free protoplasm (centrolecithal) (Fig. 11). The result of this latter dis- 

 tribution is that a skin of cells is formed over an inert mass of yolk. 

 But the clogging influence 

 of yolk extends far beyond 

 the first stages of develop- 

 ment. 



The course of development 

 can indeed be roughly divided 

 into three stages : (1) In 

 the first the zygote becomes 

 divided into a number of em- 

 bryonic cells or blastomeres ; 

 this stage is called segmen- 

 tation ; (2) in the second 

 these cells are arranged so as 

 to form the primary organs, 

 the so-called germ layers, i.e. 

 the skin, and the lining of the 

 gut and of the body cavity ; 

 this stage is called the forma- 

 tion of the layers ; and (3) 

 in the third stage these layers 

 are modified into the larval or 



y- 



n 



FIG. 11. Unripe egg of Limulus polyphemus. 

 (After Munson. ) All example of a centro- 

 lecithal egg. 



permanent organs ; this last 

 stage is called organogeny. 



ch, chorion ; g.s, nucleolus (germinal spot) ; g.v, 

 nucleus (germinal vesicle); per, peripheral cytoplusmic 

 area free from yolk ; y, central area of cytoplasm filled 

 with yolk ; //.n, yolk nucleus. 



Eggs with little or no yolk 

 are termed alecithal (Fig. 12). If yolk in the form of refringent 

 globules should be totally absent, reserve stuffs in the shape 



of masses of chroiuatm are scattered 

 about through the cytoplasm. In such 

 e <)>o 'S the building up of organs out of 



OO D J. O 



the first cells, or blastomeres which 

 result from division, takes place by the 

 simplest processes of unequally rapid 

 growth of different parts, and of folding. 

 Now in the folding of a layer of cells it 

 is essential that" the radius of curvature 

 should bear such a relation to the size of 

 the individual cell that the latter should 

 not be deformed. When the layer consists 

 of a few large yolky cells, folding becomes 

 ( After Schaxei.j An example impossible and is replaced by proliferation 



FI v % 't. * *> ^ Y'' 



FIG. 1'2. The ripe egg of 

 Strongylocentrotus lividus. 



of an alecithal egg. 



of new cells at one point in the layer. 



/(/, deposits of chromatin scat- Food, as W6 have SCCll, is USlially 



supplied to the immature ovum by the 

 sacrifice ol the less fortunate oogoma or 



immature ova. In the case of the common polyp, Hydra, the im- 

 mature egg comes at this stage to resemble an Amoeba. But in one 



