366 . WILLIAM A. HILTON 
vessel begins to develop, the ends of the perpendiculars seem to 
become rather irregular and one or two fine capillary vessels may 
be seen to connect adjacent lines before circulation. In embryos 
of about 12 mm. total length when stretched out free from the yolk 
and at a time when there is considerable pigment developed in 
the body, a vessel parallel to the lateral cutaneous connects the 
ventral ends of the perpendicular somatic capillaries which are 
somewhat variable in number, in part probably due to the stage of 
development. An early number of these is six and in larger em- 
bryos ten to eleven such vessels are found on each side (fig. 28). 
In some of these larger embryos there may be ten on one side and 
eleven on the other. Sometimes vessels connecting these perpen- 
diculars may run the whole length of the yolk without a break, but 
more usually, after about four of these near the head end, there is 
an interruption with no connection across the lower ends of two 
adjoining perpendiculars. The blood in all of these runs into the 
lateral cutaneous, but in the head end the current in the first part 
of the parallel is towards the tail, the flow coming from the region 
of the liver, while in the caudal segment of the parallel, the cur- 
rent is towards the head end. The distance between the perpen- 
diculars is somewhat variable. 
In all stages up to 12 mm., while these vessels are gradually 
being formed, the blood of the vitelline system is about as in 
younger specimens; all portions of the yolk mass, even under the 
embryo, are drained by an extensive capillary network with fre- 
quent anastomoses, and the blood flows with great rapidity in the 
large and small vessels which are mostly on the ventral part of the 
yolk. 
In larger embryos some vessels seem to a slight degree to 
extend down on the yolk beyond the limits of the body-wall, but 
I could not completely satisfy myself, even in early stages, that 
there was an anostomosis between somatopleuric and splachno- 
pleuric vessels. I believe that if there is any such anastomosis it 
is chiefly at first, and is not extensive. 
At a stage of about 12 mm. total length and a day or so before 
hatching, but at a later stage than the above, the pigment in the 
body is much more abundant and has extended down so far into 
