156 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



HECAL LAYER 



! — FOLLICLE CELLS 



ZONA RADIATA 



•PERIPHERAL 

 MITOCHONDRIAL ZON 



• - 



^^_,VARIOUS STAGES OF 

 -^^J?^ T R AN SFOR M ATION OF 

 '•^MITOCHONDRIA INTO 



Fig. 88. Portion of follicle and periphery of oocyte from ovary of the adult bird, 

 showing the mitochondria and their transformation into the M-yolk spheres of Brambell. 

 (After Brambell, '25.) 



Before considering the various aspects of cytoplasmic differentiation of 

 the oocyte, it is best for us to review the types of vertebrate and other chordate 

 eggs in order to be able to visualize the various goals toward which the 

 developing oocyte must proceed. 



1) Types of Chordate Eggs. Eggs may be classified according to the 

 amount of deutoplasm (yolk, etc.) present in the cytoplasm as follows: 



a) HoMOLECiTHAL ( IsoLECiTHAL ) Eggs. Truc homolccithal eggs in the 

 phylum Chordata are found only in the mammals, exclusive of the Proto- 

 theria. Here the deutoplasm is small in amount, and is present chiefly in 

 the form of fat droplets and small yolk spherules, distributed in the cytoplasm 

 of the egg (figs. 118A, B; 147A). 



b) Telolfcithal Eggs. In the telolecithal egg the yolk is present in con- 

 siderable amounts and concentrated at one pole. Telolecithality of the egg 

 in the phylum Chordata exists in various degrees. We shall arrange them in 

 sequence starting with slight and ending with very marked telolecithality as 

 follows: 



(1) Amphioxus and Styela. In Amphioxus and Styela from the subphyla 

 Cephalochordata and Urochordata, respectively, the yolk present is 

 centrally located in the egg before fertilization but becomes concen- 

 trated at one pole at the time of the first cleavage where it is con- 

 tained for the most part within the future entoderm cells (figs. 132D, 

 167A). 



(2) In many Amphibia, such as the frogs and toads, and also in the 

 Petromyzontidae or fresh-water lampreys among the cyclostome fishes, 

 the yolk present is greater in amount than in the preceding eggs. As 

 such, it is concentrated at one pole, the future entodermal or vegetal 



