VIVIPAROUS FISHES OF THE PACIFIC COAST. 
467 
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 entoderm 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 motion 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. 
30. 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 repre- 
sents parts of the neurenteric canal. The middle portion remains for some time as an 
enlarged 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 considerable time. 
31. The entoderm at first extends over the entire yolk. It later becomes 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 mar- 
ginal 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. 
33. The anterior opening of the alimentary canal to the exterior is through the 
gill-slit in larvae 1 mm. in length, i. e., long before the mouth is formed. The first food 
enters through this gill-slit. The food current before the fish can swallow is kept up 
by a very highly ciliated gullet which extends from behind the gill region to near the 
hind gut. 
34. The mouth does not appear till the larva has increased 3 mm., i. e., to a length 
of about 4 mm., and during all this time the hyobranchial gill-slit functions as mouth. 
There is here found a condition similar to the one supposed by Dohrn to explain the 
replacement of the annelid mouth by a gill mouth. 
35. Just in front of the notochord and near the region of the hyobranchial slit a 
strand of hypoblast cells extends up from the median portion of the alimentary tract 
to above the notochord. This strand of hypoblast cells lies in the region where Dohrn 
supposes the annelid oesophagus to have disappeared. 
36. The hind gut soon becomes enormously enlarged and later a large number of 
long villi are developed. 
37. The larvae retain as an ancestral trait a large yolk sack, the yolk being quite 
minute. The sack is largely taken up by the large pericardium through which the 
long tubular heart extends from below and behind, upward and forward. 
38. In conclusion: The fish in almost all its stages has become highly specialized. 
Many stages resemble very closely primitive conditions, but the conditions can prob- 
ably in but few cases be looked upon as a simple reversion. Its development has, on 
the other hand, become extremely ichthyized and its egg stands at the end of the 
chain of eggs in which the BrancMostoma egg, the Elasmobrancli egg, and the norma 7 
fish egg form links. 
