BULLETIN OF THE UNITED STATES FISH COMMISSION. 183 



The peculiar, homogeneous protoplasmic wall of the yelk vesicle yhy 

 persists to the last, as I have learned from sections prepared from em- 

 bryos in which the yelk sac was almost entirely absorbed. It would 

 therefore appear that the central clear portion of the yelk y was by 

 degrees transformed into the superficial palish amber layer which forms 

 the covering of the vesicle or sac. Of the forces at work in effecting 

 this transformation, we know nothing more than of the efficient cause 

 of development itself. 



Thus far we have discussed the absorption of only that part of the 

 yelk which remained after the embryo shad had* left the egg. As we 

 know that the volume of the embryo previous to hatching is greatly in 

 excess of the volume of the germinal disk, it is fair to infer that in addi- 

 tion to the mode of yelk absorption here described there must be another 

 which will account for the growth of the embryo before its heart has 

 developed enough to be an active agent in the process of yelk incorpo- 

 ration. This second method of yelk absorption has been called intussus- 

 ception, and is the primary or initial mode. It supposes that the em- 

 bryo appropriates a part of the yelk during the early stages of develop- 

 ment by a direct process of incorporation without the aid or interven- 

 tion of a blood vascular apparatus, as rudimentary even as that which 

 we have ascribed to the embryo shad. The body of the embryo, super- 

 imposed as it is upon the yelk, is supposed to derive portions of mate- 

 rial for further growth as these are needed from an " intermediary 

 layer" (Van Bambeke), which probably corresponds to our palish 

 amber yelk envelope which covers the clear yelk material in the shad. 

 This layer, called the couche hcematogene by Vogt in his embryological 

 history of Coregonus palcea, therefore appears to play an important part 

 in the development of the blood at all stages both before and after the 

 functional development of the heart. Under whatever name we know 

 it, it is undoubted that in this layer a process of cell and blood-cell 

 differentiation takes place. This statement is grounded on two sets of 

 facts ; namely, the observation of free nuclei in this layer by embryol- 

 ogists, and the undoubted circumstance of the origination of blood cells 

 from its surface. Blood cells, especially white ones, are known to be 

 nucleated, and no others are at first formed in the shad ; it therefore 

 follows that the nucleation must occur in the layer here understood. 

 Kupffer* has alluded to a similar process, but from what I have been 

 able to gather from his writings he does not seem to have been clear 

 in his understanding of the layer, confounding it with the true hypo- 

 blast. This opinion I was also led to adopt in my essays on the 

 Spanish mackerel and silver gar, but I am now in doubt whether 

 this view can be justified. My main reason being that I have been 

 unable to discover any evidence that the intestine of the shad origi- 

 nates from this yelk envelope in sections prepared from such stages 



*Beobaclitungen liber die Eutwickelung tier Knocheufische. Arch, fiir Mik. Anat., 

 iv, 1868. 



