YOLK FORMATION 147 



by a reference to its composition and origin. As a matter of fact, 

 the terminology suggested by him at the suggestion of Gatenby 

 refers only to the origin, i.e., Golgi yolk, nucleolar yolk, etc. 

 This in our opinion is unsatisfactory, as it prejudges the case. 

 Further, the only criteria which are more or less independent of 

 personal observation and opinion are chemical ones and therefore 

 refer to the composition of the yolk. This has apparently been 

 found the case in practice, and the literature is confused by terms 

 such as " true yelk " or " ordinary yolk," the former being used 

 by Gatenby (33) for light fatty yolk in Saccocirrus, the latter by 

 Bowen (11) to refer to heavy albuminous yolk in general. Bram- 

 bell produces support for his suggestion by observations on 

 Helix and Patella. In both these molluscs there are two types of 

 reserve product ; one is light (as shown by centrifuging experi- 

 ments), fatty and produced by the Golgi apparatus (vide Gatenby 

 above), the other is heavy, albuminous and formed by a swelling 

 of the mitochondria. Differences between Helix and Patella 

 consist in the fact that the fatty yolk is formed directly /rom the 

 Golgi apparatus in Helix, but rather secreted by the Golgi in 

 Patella. There are also differences in the proportions of the two 

 types of yolk. He can find no evidence as to how the mitochon- 

 dria swell up, whether by building up material themselves or by 

 absorbing it from the cytoplasm. The metamorphosis of the 

 Golgi rods in Helix is shown by all the intermediate conditions 

 which he finds, and it is produced by a deposition of material 

 within their own substance, presumably without loss of identity 

 by the latter. This brings it more or less into line with Patella. 



Bowen reviews the position in 1926 (11), and although few new 

 facts had been produced, is inclined to the view that the whole 

 situation w^as in a hopeless confusion. This attitude will probably 

 be inevitable if too much weight is put upon the earlier workers' 

 results, for, not merely improved technique, but also a general 

 clarifying of ideas has taken place and more exact terminology is 

 now possible, although it is not resorted to too often. Bowen 

 suggests that there are two and only two kinds of yolk. Firstly 

 fatty yolk ; lipoidal in nature, blackened by osmic acid, light and 

 oily and therefore easily found at the top of the egg after centri- 



