ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



of the original germinal cell, while the yolk is only secondarily 

 developed with the gradual growth of the first ; and not unfrequently 

 it is derived from the secretion of special glands (yolk glands, Trema- 

 todes) ; it may even be added in the form of cells. 



In the Ctenophora and other Crelenterata we see already in the 

 first-formed segments the separation of the formative matter or 

 peripheral ectoplasm from the nutritive matter or central 

 endoplasm. 



In eggs undergoing a partial segmentation the formative matter 

 usually lies on one side of the large unsegmenting food yolk. In 

 accordance with this, the segments of such eggs, known as telolecithal, 

 arrange themselves in the form of a flat disc (germinal disc) hence 

 this kind of segmentation has been called discoidal (eggs of Aves, 

 Reptilia, Pisces) (fig. 105). The food yolk may, however, have a 

 central position. In such centrolecithal eggs the segmentation is 



FIG. 106. Unequal segmentation of the centrolecithal egg of Gamrnarus locusta (in part after 

 Ed. van Beneden). The central yolk mass does not appear till a late stage and undergoes 

 later an " after-segmentation." 



confined to the periphery, and is sometimes equal (Palsenion) and 

 sometimes unequal (fig. 106). The central yolk mass may at first 

 remain unsegmented, but later it may undergo a kind of after- 

 segmentation and break up into a number of cells (fig. 106). Again, 

 in other cases the food yolk, at the commencement of segmentation, 

 has a peripheral position, so that the cleavage process is at first 

 confined to the inner parts of the egg, and only in later stages, when 

 the food yolk has gradually shifted into the centre of the egg, 

 appears as a peripheral layer on the surface. This is found especially 

 in the eggs of Spiders (fig. 107). The first processes of segmentation 

 in these at first ectolecitlial ova are withdrawn from observation, 

 since they take place in the centre of an egg covered by a superficial 

 layer of food yolk, until the nuclei with their protoplasmic invest- 



