192 Proceedings of the Royal Physical Society. 



toplasm which is not included in the germinal disc. To 

 take, again, the example of Trachinus, the collection of 

 protoplasm at the upper pole (the lower as the egg floats) of 

 the egg may be conveniently termed the germinal mound or 

 germinal area, as the whole of this protoplasm is not included 

 in the germinal disc, that is to say, in the archiblast. When 

 this becomes defined, there is still left a thin layer of the 

 original germinal protoplasm around the yolk, which is the 

 parablast. If the whole of the germinal protoplasm were 

 included in the archiblast, it is clear that any absorption of 

 nutriment which could take place through the cell- wall would 

 be much too slow a process for the nourishment of a rapidly - 

 increasing organism. So long as there is a layer of naked 

 protoplasm around the yolk, intracellular digestion may take 

 place. As a matter of fact, the cortical protoplasm does 

 include particles of yolk material within its substance, and 

 digests them there. In this manner the food material is 

 assimilated. It thus becomes the special function of the 

 parablast to act as an intermediate digestive area between 

 the segmenting archiblast and the passive food-yolk. The 

 importance of this layer has already been recognised by 

 Waldeyer and others in the development of the chick and 

 other forms. It is only amongst the students of Fish Em- 

 bryology that the full significance of the layer is not recog- 

 nised. The protoplasm elaborated from the yolk must of 

 necessity take part in the formation of the embryo. The 

 continued subdivision of the archiblast cannot go on without 

 nourishment, and as a layer of unsegmented protoplasm exists 

 between it and the food supply, it is clear that nourishment 

 can only be derived through the agency of the parablast. 



As Waldeyer has already pointed out, the cells which are 

 derived from the parablast are to be regarded as secondary 

 segmentation products, whose modification is connected with 

 the supply of food-yolk. In what manner the cells thus 

 derived share in the economy of the embryo, does not con- 

 cern us here. On a jpriori grounds, and judging from the 

 large supply of food material, one would be led to conclude 

 that it must be no unimportant one. It has been asserted 

 by Hoffmann that the parablast cells do not take any part in 



