482 j. bronte gatenby 



5. General Account of the Yolk Formation in Birds 



AND Amphibia. 



In both birtls and amphibians the egg is richly provided 

 with yolk, i. e. macrolecithal. The formation of yolk in the 

 egg of amphibians does not seem to have been followed out 

 with any detail or pains by a modern worker, and though 

 I have made numerous preparations by the best methods, it has 

 been difficult to determine the exact source of origin of the yolk- 

 spheres (see Gatenby, 15, p. 139). 



In the amphibian oogenesis the mitochondria spread out 

 mainly to form a deep cortical zone on the periphery of the 

 egg. It is in this zone that the first sign of yolk-granules 

 appears, but it is wellnigh impossible to give an opinion as to 

 whether the yolk originates directly from the mitochondria, 

 or whether the latter only elaborate materials which, precipitat- 

 ing in the ground cytoplasm, come to form the separate spheres 

 of substance we recognize as yolk. 



Van Dunne, in his monumental work on the oogenesis of 

 birds (10), has entered into the subject with care, and has 

 produced a paper which may be accepted as an authentic 

 account of the steps in the formation of yolk in birds. He 

 recognizes in the oocyte just before the beginning of the exten- 

 sive yolk formation : (1) an attraction sphere containing 

 a centrosome, (2) a yolk-forming region or vitellogeiious cloud, 

 (3) a quantity of fatty yolk. The vitellogenous cloud is formed 

 of mitochondria of various types, e. g. chondriomites, chondrio- 

 somes, and it soon undergoes a process of dissociation. This 

 dissociation of the ' couche vitellogene ' invokes the appearance 

 throughout the egg-cytoplasm of a uniform layer of mito- 

 chondria. This miiformity does not last long, for soon after- 

 wards three distinct mitochondrial zones appear ; a cortical 

 dense zone, an inner deeper, and an internal still deeper zone. 



The first vestiges of yolk formation are the appearance 

 of clear yolk-vesicles (vacuoles) in the neighbourhood of the 

 cortical fatty layer, thus constituting a peripheral vacuolated 

 area, which spreads gradually towards the centre of the yolk ; 



I 



