INTRODUCTION 



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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 

 permanent organs ; this last 

 stage is called organogeny. 



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 chromatin are scattered 

 about through the cytoplasm. In such 

 eggs the building up of organs out of 

 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 

 impossible and is replaced by proliferation 

 of new cells at one point in the layer. 



Food, as we have seen, is usually 

 supplied to the immature ovum by the 

 sacrifice of the less fortunate oogonia 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 



Fia. 11. Unripe egg of Limulus polyphemus. 

 (After Munson.) An example of a centro- 

 lecithal egg. 



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

 nucleus (germinal vesicle) ; per, peripheral cytoplasmic 

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

 with yolk ; y.n, yolk nucleus. 



FIG. 12. The ripe egg of 

 Strongylocentrotus lividus. 

 (After Schaxel. ) An example 

 of an alecithal egg. 



ehr, deposits of chromatin scat- 

 tered through the cytoplasm and act- 

 ing as reserve material ; n, nucleus. 



