Relation of Yolk to Blastoderm in Teleostean Fish Ova. 191 



vegetative area, morphologists have sought to explain the 

 partial segmentation of meroblastic ova as an extension of 

 the same retarding process. It is usually stated that in a 

 meroblastic ovum, such as that of the cod, the first furrow 

 theoretically divides the ovum into two equal parts, but that 

 practically this furrow is incomplete on account of the over- 

 whelming preponderance of food material in the lower pole. 

 To my mind such a statement is misleading ; it presupposes 

 an analogy which does not exist. It is the difference in the 

 distribution of the yolk in the two cases which destroys the 

 analogy. If the yolk-pole of the cod ovum only differed 

 from that of an amphibian in containing an enormously 

 greater proportion of yolk, the analogy might hold good. 

 The difference, however, is far greater. The yolk-pole con- 

 sists of one large sphere of passive food material practically 

 void of protoplasm, around which there is a thin layer of 

 protoplasm, which is part of the germinal area. The outlines 

 of the two, however, are well defined. 



To return to the analogy of the fat -cell. If a cell, con- 

 taining within its protoplasm a large fat vesicle, were to 

 divide and give rise to two daughter cells, no one would 

 suppose for a moment that the line of cleavage passed theo- 

 retically through the centre of the fat vesicle. I submit that 

 the case of the cod ovum is precisely analogous. I am aware 

 that in some fish ova the separation of protoplasm from yolk 

 is not so complete as is the case in the pelagic group of ova. 

 The protoplasm and the yolk are, however, distinct from 

 each other, and it is only because certain protoplasmic pro- 

 cesses are pushed in amongst the yolk spheres that the active 

 and passive portions of the egg are brought into closer union. 

 It appears to me, therefore, that there is no benefit to be 

 derived by theoretically regarding any furrow as passing 

 through a mass of passive food material, which is of no use 

 to the embryo until it has become converted into active 

 living protoplasm. This brings us to the question, How does 

 the food-yolk become utilised in the fish ovum ? My answer 

 is, through the agency of the paraUast, and probably through 

 that agency alone. I have previously defined the parablast 

 as primarily consisting of that portion of the germinal pro- 



