MATURATION OF GAMETES 157 



pole, and a greater degree of telolecithality is attained than in the eggs 

 of Amphioxus or Styela (fig. 141 A). 



(3) In many Amphibia, such as Necturus, also in Neoceratodiis and Lepi- 

 dosiren among the lung fishes, and in the cartilaginous ganoid fish, 

 Acipenser, yolk is present in considerable amounts, and the cytoplasm 

 of the animal pole is smaller in comparison to the yolk or vegetal 

 pole (figs. 150, 151, 152). 



(4) In the bony ganoid fishes, Amia and Lepisosteiis, as well as in the 

 Gymnophiona (legless Amphibia) the yolk is situated at one pole 

 and is large in quantity (figs. 153B-F; 154). 



(5) Lastly, in a large portion of the vertebrate group, namely, in reptiles, 

 birds, prototherian mammals, teleost and elasmobranch fishes, and 

 in the marine lampreys, the deutoplasm is massive and the proto- 

 plasm which takes part in the early cleavages is small in comparison. 

 In these eggs the yolk is never cleaved by the cleavage processes, and 

 development of the embryo is confined to the animal pole cytoplasm 

 (figs. 46, 47). 



2) Formation of the Deutoplasm. The cytoplasm of the young oocyte is 

 small in quantity, with a clear homogeneous texture (figs. 68A; 86A, B). As 

 the oocyte develops, the cytoplasmic and nuclear volumes increase (fig. 68F), 

 and the homogeneity of the cytoplasm is soon lost by the appearance of 

 deutoplasmic substances (fig. 68G, H). In the oocyte of the frog, for example, 

 lipid droplets begin to appear when the oocyte is about 50 /x in diameter 

 (fig. 72A). (See Brachet, '50, p. 53.) A little later glycogen makes its 

 appearance, and finally yolk protein arises. 



The origin of fat droplets and yolk spherules has been ascribed variously 

 to the activities of chondriosomes (mitochondria and other similar bodies), 

 Golgi substance, and of certain vacuoles. Most observers place emphasis 

 upon the presence of a so-called "yolk nucleus" or "yolk-attraction sphere" 

 situated near the nucleus of many oocytes as a structure associated with fat 

 and yolk formation. In general, two types of yolk bodies have been described. 

 One is the yolk nucleus of Balbiani and the other the mitochondrial yolk 

 body of Brambell. The yolk nucleus of Balbiani (fig. 86A, B) consists of 

 the following: 



( 1 ) a central body, the centrosphere or archoplasmic sphere within which 

 one or more centriole-like bodies are found, and 



(2) surrounding this central body, a layer of Golgi substances and chon- 

 driosomes (i.e., mitochondria, etc.). 



This cytoplasmic structure probably is related to the idiosome of the oogonia 

 (fig. 68A). 



The formation of the deutoplasm, according to the theory associated with 

 the Balbiani type of yolk nucleus is as follows: The surrounding pallial layer 



