Mitocliondria, Golgi Apparatus, and Yolk. 147 



Later tlie yolk nucleus (or nuclei) disappear without leaving a 

 trace. (See also page 139.) 



Sufficient work has not been carried out in the light of the 

 newer interpretations of the inclusions of the cytoplasm to enable 

 us to take a satisfactory survey of the conditions leading to the 

 formation of the " yolk nuclei " in forms such as the arthropod or 

 the vertebrate. In mammals the archoplasm has long been known 

 as the " yolk body of Balbiani," but probably in no mammal does 

 the archoplasm form true yolk. The " yolk nucleus " of the spider 

 egg or the ascidian appears to be different from the archoplasm. 



Graphic Eepresentation of the Formation of the 

 "Yolk" in Various Forms. 



In the following section we have attempted to represent 

 graphically the manner of formation of the yolk and associated 

 bodies in several forms. We recognize several grades of progres- 

 sive complexity. At one end we find such a case as Grantia, where 

 the yolk is formed as vacuoles in the ground cytoplasm, and where 

 neither mitochondria nor Golgi apparatus take direct part in the 

 production of yolk ; at the other end we find such an example as 

 the ascidian, where the formation of the '' yolk " is extraordinarily 

 complex. Intermediate forms are found in Patella, and possibly 

 Bana. Such a mammal as Lepus is specialized and simplified : 

 the mitochondria may contribute to the formation of fat, while 

 the Golgi apparatus does not appear to be altered. 



These grap)]iic representations are based on observations which 

 in some cases are at 'present difficult to prove or disprove, and to 

 lohich future work may lend a different interpretation. In each 

 case we begin in the oogonium with Golgi elements and mito- 

 chondria, and by the time the oocyte has become full grown the 

 latter bodies may metamorphose in a most complicated manner, or 

 they may simply have grown in bulk or in the number of their 

 individual parts, without having become loaded with food sub- 

 stances which would cause them to be classified as " yolk." 



PORIFERA. 



Grantia compressa. 

 Oogonium. Full-grown Oocyte. 



Mitochondria — — ^ Mitochondria. 



(Ground cytoplasm) >- Yolk spheres. 



Golgi apparatus — --> Golgi apparatus. 



L 2 



