61 



toderm from ectoderm and is completely diplastic and symmetrical, the 

 blastofore closing at the entodermic pole of the egg. 



26. Before any other organs become evident the sex cells become con- 

 spicuous. Their fate I have discussed elsewhere. 



27. The earliest stages of the formation of the embryo have not been 

 clearly made out with the material at hand. It is, however, certain that 

 in one of the figures published by me in the " Journal of Morphologv,'" I 

 mistook the tail for the head. The conditions are extremely similar to 

 those found in the mammalian embryos, except that the central cavity is 

 filled with yolk instead of fluid. 



28. The mesoderm is formed by a process of delamination from the 

 entoderm. It is formed as two sheets and over the whole of the ento- 

 derm exclusive of the axial line. 



29. The young fish is freed from its membrane in a very immature 

 condition. It completely encircles the yolk ; in fact the head and the 

 tail overlap. It is incapable of motiou at this time and indeed the cells 

 which will form the muscles have scarcely become differentiated. The 

 hatching process is due to the growth of the embryo and not to its activity 

 as is usually the case. The fin folds do not appear till much later. 



.'JO. Kupffer's vesicle appears very early and is very large. It consists 

 when fully formed of a dome-shaped roof over a large cavity surrounded 

 on the sides by entoderm. It at first rests on the yolk but soon the yolk 

 is forced down and presents a deep impression just beneath the vesicle. 

 Later the vesicle is divided into three distinct cavities. The upper dome- 

 shaped portion persists for some time and probably represents part of the 

 neurenteric canal. The middle portion remains for some time as an en- 

 larged part of the intestine. The lowest portion is the cavity formed in 

 the yolk. It has acquired a roof by the ingrowth of the entoderm cells 

 to form the floor of the intestine. This cavity usually remains for a con- 

 siderable time. 



31. The entoderm at first extends over the entire yolk. It later be- 

 comes restricted to a comparatively narrow strip along the axial line. 



32. The floor of the alimentary canal is formed by the ingrowth below 

 of the marginal cells of the entoderm. The ingrowth progresses from in 

 front back. A lumen is not formed at once. The lumen is formed in 

 the hind gut and in the gill region at the same time and gives 

 abundant evidence that the alimentary tract is bilateral. The middle 

 anterior part remains a solid mass of cells after the lumen has appeared 

 both in front and behind this tract. 



