94 



GENERAL EMBRYOLOGY 



which is entirely free from yolk. Between these extremes all 

 intermediate conditions are found. This telolecithal type of 

 egg is very common among the Invertebrates, and is charac- 

 teristic of all the Craniata except the true Mammals. Among 

 the Chordata successive stages in the accumulation of yolk are 

 represented by Amphioxus, Lampreys, Ganoids, Dipnoans, 

 Amphibians, Reptiles, and Birds. In the last two groups the 



d 



FIG. 48. Egg of the Teleost, Fundulus heteroclitus. Total view, about an 

 hour after fertilization, c, chorion; d, protoplasmic germ disc or blastodisc; 

 o, oil vacuoles; p, perivitelline space; v, vitelline membrane; y, yolk. 



protoplasm is extremely limited in amount, and is found only 

 as a small disc or layer on the surface of the spherical yolk- 

 mass, at the animal pole. A third and less common arrange- 

 ment of yolk is that seen in the centrolecithal eggs of many 

 Arthropods, chiefly Insects. Here the yolk occupies a greater 

 or lesser portion of the center of the egg while the protoplasm 

 forms a superficial layer all around it (Figs. 47, 117, 11$). 



