GERM CELLS AND THEIR FORMATION 93 



In addition to the formative substances mentioned above 

 eggs may contain varying amounts of nutritive substance of 

 many different kinds, collectively termed yolk oT_deutoplasm. 

 The yolk may be in the form of granules, small spherical bodies, 

 large plates, fluid drops of various sizes, or in compact masses 

 (Figs. 45, 48). These substances may be of different chemical 

 compositions and staining reactions in a single egg. They 

 may be formed within the egg by its own activity, or they may 

 be contributed indirectly by cells associated with the egg during 

 its formation. The arrangement of the food substances in the 

 egg has an important bearing upon its later development, espe- 

 cially upon the form of its cleavage (Balfour). Eggs in which 

 the yolk is distributed quite uniformly through the cytoplasm, 

 and in which the protoplasm is therefore more or less com- 

 pletely intermingled with the yolk granules, or plates, are 

 termed homolecithal or isolecithal eggs. Some eggs have been 

 described as alecithal, i.e., without yolk, but many of these 

 have been found really to contain a small amount of quite 

 uniformly distributed deutoplasm, and a truly alecithal egg is 

 rarely if ever found. Eggs of some species among nearly all 

 the large groups are of this homolecithal type, for example, 

 the star-fish, sea-urchin, and also the Mammals, which were 

 formerly thought to be alecithal (Fig. 43). More frequently 

 the yolk and cytoplasm are not uniformly mingled but are 

 chiefly accumulated in different parts of the cell. Ordinarily 

 these materials occupy opposite poles of the egg so that this 

 retains a radial or rotatorial symmetry; the yolk is accumulated 

 toward the vegetative pole, the protoplasm toward the animal 

 pole|(Fig. 48). Such eggs are termed telokcithal. They show 

 great variation in the relative amount of yolk contained. On 

 the one hand it is often difficult to distinguish the telolecithal 

 egg from the homolecithal type, for the tendency toward polar 

 accumulation of the yolk may be very slight. The egg of 

 Amphioxus illustrates such a transitional condition. At the 

 opposite extreme we find eggs such as those of the Reptiles and 

 Birds, which are relatively immense cells, in .which it is difficult 

 to distinguish, before development begins, any definite region 



