686 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



protoplasmic network, such as the reptilian ovum presents, or as Dr Schultz demonstrated 

 (No. 144) in the Selachians. 



Little or no food-yolk makes its way into the germinal area, so that, as Lereboullet 

 observes (No. 93, p. 485), it takes no part in the segmentation of the germ. Indeed, all 

 evidence tends to prove that the deutoplasm is in an inert or quiescent state, and only 

 passively contributes to embryonic development, being slowly incorporated by the active 

 protoplasm of the blastodisc in a mode which Ryder compares to the process of ingestion 

 and assimilation in Amoeba (No. 141, p. 557). 



When the eggs of Gadus morrhua are partially dried, the surface of the yolk shows 

 a series of clear reticulations, which on re-immersion in water run together and disappear 

 in the course of eight or ten minutes ; such reticulations have, however, no connection 

 with the later protoplasmic reticulation of the vitellus after epibolic extension of the 

 blastoderm, and which is very noticeable in the cod, common dab (PL V. figs. 3, 11), 

 and others. Haeckel regards it as so much passive matter contained in a gastrula- 

 cavity (No. 62) ; but in Teleosteans it plays a more important role in later stages than 

 that of supplying crude pabulum to the germ. Indeed, the germinal protoplasm 

 Balbiani holds to be solely transformed yolk — not a mere segregation of interfused 

 germinal matter. The germ, he says, is formed "by endogenous development of cells at 

 the expense of the yolk or primordial protoplasm;" but he repeats the error of Coste 

 that the germinal area is never formed until after fecundation (No. 9). 



J. T. Cunningham, in a highly suggestive paper, observes that the yolk and germ are 

 equally concerned in the processes of cleavage ; segmentation in Teleosteans (as in 

 Amphibians), dividing the ovum at the first stage of cleavage not equatorially, as E. 

 van Beneden holds (No. 25), but meridionally into two similar halves, each with a cap of 

 protoplasm and a mass of subjacent food-matter (No. 48). This view, however, gives 

 to the crude deutoplasm an importance which cannot be accorded to it, even though 

 cleavage as regards the yolk be merely potential and never fully achieved. 



The separation of the deutoplasmic mass into a segmenting blastoderm (PL XXII. 

 fig. 1, bd), and an appended ball of pabulum (Ibid., y), is more complete in osseous fishes 

 than in Elasmobranchs, and imparts to the yolk rather an accessory character than that 

 of an active participator in the whole process of cleavage. 



That it contributes to the growing organism, and even buds off cells to build up part 

 of the enteric tract, does not conflict with this view, which is supported by the fact that 

 the yolk persists as a bulky appendage on the ventral surface of the young fish (PL XIX. 

 figs. 5, 7), until a late embryonic stage, being enclosed by the body- wall, and finally 

 absorbed when the post-larval stage is reached. The passive role attributed to the yolk 

 Ryder would confine to the early stages, while later its function, he holds, is more 

 important, since it becomes through the medium of the intermediary layer an active part 

 of the ovum (No. 141, p. 569). 



But this view is not inconsistent with that here maintained, for if it serves as pabulum, 

 this is really a part secondary to actual participation in blastodermic cleavage, and while 



