VIVIPAROUS FISHES OF THE PACIFIC COAST. 
451 
tube extending from the posterior wall of the pericardium upward and forward; the 
yolk lies in the sinus venosus. About 22 protovertebrae are formed. 
There is little change in general outlines for some time after a length of 1 mm. is 
reached. The tail elongates and fin folds are developed. One striking feature is that 
the fin-fold is very narrow at the tip of the tail, while the dorsal and caudal portion 
may be well developed. 
The segmentation of the hind brain (fig. 93) becomes very prominent when the larva 
has reached a length of 2 mm. and remains so till the larva is 5 or more mm. long. 
During this period the hind brain (figs. 91, 93, 96) is divided by lines which are con- 
spicuous during life, and quite evident in preserved specimens. 
In larvae 5 mm. long (figs. 94 and 95) the notochord extends to the tip of the tail, 
and a caudal fin-fold proper is not developed; the dorsal and anal folds are present. 
The posterior part of the anal fold shows a thickening where the caudal is finally 
formed. The pectoral has also made its appearance as a small flap on the shoulder. 
The liver has become well developed and fills a part of the space formerly occupied 
by the yolk, some of which still remains. The heart is still a simple tube without any 
differentiations. The intestine has become much enlarged, and in the protruding hind 
gut, which is distinctly differentiated, ridges have been formed on the inner side, which 
later become transformed to villi. The whole tube is still simple at this time without 
any loops. 
The circulatory system (fig. 92) consists of an arch extending upward and forward 
from the heart, thence abruptly backward to beyond the middle of the tail, forward to 
near the anus, downward to the lower surface of the intestine, along which it extends 
till before reaching the yolk it bends upxvard around the intestine and then down over 
the yolk and into the heart. Shortly afterwards the intestine bends down at the 
vascular loop and forms the familiar sigmoid curve (figs. 96-100). By this means the 
portal vein runs straight forward along the ventral surface of the hind gut and 
then directly to the liver. The succeeding changes in the relative size and the curves 
of the various parts of the intestine may be gathered from figs. 93a to 93e. 
Concomitant with the changes in the intestine the simple tube of the heart is 
transformed into auricle, ventricle, and bulbus arteriosus. While it is a simple tube 
it arises from the sinus venosus near the bottom. This origin is translocated upward 
till it arises from the top of the sinus. At the same time the tube has beeu lengthened 
and two constrictions have appeared. The posterior section comes to lie over the 
middle section; finally the upper is moved forward, and by the elongation of the 
bulbus the middle section is pushed backward so that the relation obtaining in figs. 
96-99 results. The definite fins appear when the larva has reached a length of about 9 
mm. (fig. 99). The further externally visible changes maybe gathered from the accom- 
panying figures (101-108). Pigment does not make its appearance till very late, and 
then only sparingly, except in the eye. 
Formation of the intestine . — The fundament of the intestine can first be distin- 
guished in my sections when three protovertebrae are formed and several others are out- 
lined (figs. 67-74). The conditions are similar in a number of embryos sectioned, and 
1 have selected for description one sectioned longitudinally and another transversely. 
In a section in the sagittal plane (fig. 67) the line separating the chorda from the 
neural thickening is well marked. The line separating the chorda from the under- 
lying hypoblast is quite distinct in the anterior part of the embryo where the chorda 
