DESCRIPTION OF THE SEXUAL PRODUCTS. 



11 



cleavage, make their appearance only at the region of the egg 

 which is richer in protoplasm, whereas the region which is more 

 voluminous and richer in deutoplasm remains apparently quite 

 unaltered, and is not divided up into cells. By this means the 

 contrast, which was already present in the unsegmented egg, 

 becomes during development disproportionately greater and more 

 obvious. The one part undergoes changes, is divided into cells, and 

 out of these produces the individual organs ; the other part remains 

 more or less unaltered, and is gradually employed as nutritive 

 material. Following the example of REICHERT, the part of the 

 yolk which is richer in protoplasm, and to which the developmen- 

 tal processes remain confined, 

 has been designated formative 

 yolk, and the other nutritiv^ 

 yolk. 



The unequal distribution of 

 formative yolk (vitellus forma- 

 tivus) and of nutritive yolk 

 (vitellus nutritivus) within the 

 egg is accomplished in two dif- 

 ferent ways. 



In the one case (fig. 3) the 

 formative yolk is accumulated 

 at one pole of the egg as &,flat 

 germ-disc (k.scJi). Inasmuch as 

 its specific gravity is less than 

 that of the nutritive yolk (n.d) 

 collected at the opposite pole, it 

 is always directed upward, and 

 it spreads itself out on the yolk just like a drop of oil on water. In 

 this case, therefore, the egg has undergone a polar differentiation ; 

 when at rest it must always assume a definite position, owing to the 

 unequal weight of the two poles. The dissimilar poles are distin- 

 guished : the upper, lighter pole, with the germ-disc, as the animal 

 (A.P}; the under, heavier and richer in yolk, as the vegetative pole 

 (V.P). The polar differentiation of eggs is often encountered in 

 Vertebrates, and is especially prominent in the classes of Bony 

 Fishes, Reptiles, and Birds. 



In the second case (fig. 4) the formative yolk (b.d) is accumulated 

 over the whole surface of the egg, and surrounds the centrally placed 

 nutritive yolk (n d) as a uniformly thick, finely granular cortical 



V.P 



Fig. 3. Diagram of an egg with the nutritive 

 yolk in a polar position. The formative 

 yolk constitutes at the animal pole {A.P) a 

 germ-disc (k.sch), in which the germinative 

 vesicle (k.b) is enclosed. The nutritive yolk 

 (n.d) fills the rest of the egg up to the 

 vegetative pole (.V.P), 



