ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 711 



Embryo Fishes.* — The Bulletin of the United States Fish Com- 

 mission contains a series of articles upon various matters connected 

 with the development of fishes, embodying the results of the investiga- 

 tions of Mr. J. A. Eyder during the year 1882. 



The mode of absorption of the yolk of the embryo shad differs in 

 the absence of a vitelline circulation from that which obtains in 

 Ti/losurus {Belone), Fiindulus, Esox, and Salmo. The great mass of the 

 yolk in the shad embryo consists of coarse irregular masses of very 

 clear protoplasmic matter, separated by a protoplasm which is opti- 

 cally different. The covering of the yolk is a palish amber-coloured 

 layer, quite different from the clear body of the yolk, and usually 

 thicker at the end next the heart. The intestine lies in a longitudinal 

 furrow on the dorsal aspect of the yolk-sac, and is never connected 

 with it in this species. The yolk-sac is surrounded by a space 

 filled with serous fluid. This space is capacious anteriorly, between 

 the heart and the yolk, and this part is identified by the author 

 with the segmentation-cavity. The delicate pericardial membrane 

 that separates this cavity from the pericardial space may, possibly, be 

 perforated. In Tylosurus the two cavities are certainly connected. 

 The heart opens freely into the segmentation-cavity, and the appear- 

 ance presented is that its persistent pulsation breaks up the yolk- 

 substance into small spherules, sucks them out of the segmentation- 

 cavity, and carries them into the body of the embryo. The corpuscles 

 develope on the surface of the outer yolk-layer, and after a while drop 

 into the serous fluid, appearing like the white blood-cells of human blood. 

 As development proceeds, the yolk-sac becomes pointed in front, and 

 the external layer becomes thicker, while the pericardial membrane 

 becomes funnel-shaped to fit the anterior part of the yolk-mass. Before 

 the final disappearance of the yolk, the liver of the young fish becomes 

 more developed, and the portal vein makes its way over the dorsal 

 aspect of the yolk towards the venous end of the heart. As the 

 peculiar amber-layer around the yolk persists to the last, it is probable 

 that the central clear portion is transformed gradually into it. 



This is the history of the yoik-mass after the embryo is hatched, 

 but as it grew in size before hatching, yolk-absorption must have 

 taken place before the heart was sufficiently developed to be an active 

 agent in the process. This must be by intussusception, and in the 

 amber yolk-covering it is undoubted that a process of cell and blood- 

 cell differentiation takes place. Mr. Ryder concludes that the hypo- 

 blast of Gensch, said by that investigator to be the source from which 

 the blood is derived, is the equivalent of the amber yolk-covering of 

 the shad, and not the true hypoblast. This amber layer is a tempo- 

 rary structure, which disappears entirely, and does not enter into the 

 formation of any organ or membrane. The serous cavity around the 

 yolk in the shad represents the body-cavity, and the outer covering 

 of this, though only 1/2000 of an inch in thickness, contains epiblast, 

 mcsoblast, and hypoblast. 



* Bull. U.S. Fish Commission— Observations on the Absorption of the Yolk, 

 the Fo(k1, F'eeding and Development of Embryo Fishes, &c., pp. 179-205. Amur. 

 Nfttural., xviii. (1HH4) pp. 3ii5-8. 



