BULLETIN OF THE UNITED STATES FISH COMMISSION. 185 



I have elsewhere* alluded to the researches of Gensch,t who investi- 

 gated the development of the blood of Zoarees and Esox. lie observes 

 that the blood originates in these forms by budding from the hypoblast, 

 and credits Kuplier with having been the first to call attention to the 

 fact. I cannot help thinking, however, that what he means by the 

 hypoblast is really the equivalent of the palish amber envelope of the 

 yelk of the shad, and in no sense anything but a temporary and evanes- 

 cent structure, which vanishes completely when the contained yelk mate- 

 rial has been absorbed. It may be proper, perhaps, to designate this 

 structure by the name of yelk hypoblast, but beyond the name it is doubtful 

 whether it is proper to imply more, because I have yet to learn, after 

 careful investigation, that it ever enters into the formation of any of the 

 organs or membranes of the body cavity in which it is actually inclosed. 



Before concluding, however, I wish to call attention to one more differ- 

 ence between the embryo of fishes with a vitelline circulation and those 

 without it. Iu those forms in which the blood-vascular network cover- 

 ing the yelk is well developed the hypo-blastic vascular layer is relatively 

 thick and distinct in cross-sections. In those in which there is no vitel- 

 line circulation the reverse is the case. When w r e come to examine cross- 

 sections, the epiblast, mesoblast, and the true hypoblast are so intimately 

 united and their combined thickness so slight that it is with great diffi- 

 culty that they are resolved wdth the microscope. In the young shad, 

 directly after hatching, the outer covering of the yelk is extremely thin, 

 and measures about 5 0V0 °* an inch in thickness. Immediately beneath 

 it and separate from it lies the homogeneous wall of the yelk vesicle. 

 This structure, which we have chosen to call the yelk hypoblast above, 

 is, on the contrary, often ten times as thick as the outer and external 

 yelk envelope which comprises, as we saw, all of the embryonic layers, 

 but which have been reduced to the greatest tenuity. 



From the foregoing recital of facts we are led to a somewhat clearer 

 understanding of the method of yelk absorption as observed in young- 

 fishes. We cannot help admiring the simplicity and efficiency of the 

 apparatus. Whether the space identified by me as the segmentation 

 cavity in fishes must be considered equivalent to the pleuroperitoneal 

 space in the embryos of birds, I am unable to state ; this is, however, 

 probable. Practically, there is very little difference between the mode 

 of yelk absorption, as manifested in the chick and in the fish. If by a 

 large license, as it seems to me, we admit that the yelk vesicle of the 

 shad is really its hypoblast, the origin of the blood and the incorpora- 

 tion of the yelk substance are similar in birds and fishes. In the latter 

 the nature of the hypoblast may be so obscured that I may have fallen 

 into error in not regarding the yelk wall as hypoblastic; however, that 



*Notes on the development, spinning kaliitsand structure of the four-spined stickle- 

 back Apeltea quadroons. Bull. U. S. Fish Commission, 1881, pp. 24-29. 



tDie Blutbilduug auf dem Dottersack bei Knockeurisclien. Arch, fur Mik. Anat., 

 xix, pp. 134-136. 



