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 centroledthal 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, 118). 



