378 WILLIAM A. HILTON 
' the last somatic set which develops as the body-wall grows about 
the yolk. 
In Amblystoma the ventral abdominal cannot be easily traced 
because of so much pigment on the outside; in sections the body- 
wall has, even in an early stage, grown down quite a distance and 
contains large blood spaces, some of which remain as cutaneous 
vessels, others become transformed into the ventral abdominal. 
Hochstetter compares the formation of the ventral abdominal 
to the allantoic vessels of higher vetebrates. These capillaries 
which migrate in this way about the yolk mass may give some indi- 
cation of how such vessels originated or how they might have been 
formed, even though the vessels themselves may not exactly 
correspond. 
The reason for the development in stages of perpendicular and 
parallel systems is evidently due to the gradual overgrowth of the 
body-wall upon the yolk. This may explain why such a progress 
is not so striking in Amblystoma. The development of systems 
of parallels is probably due to different periods of overgrowth 
when the advancing edge becomes thick enough for a vessel. 
As the somatic system is increased the vitelline becomes more 
and more diminished; some of the functions of the vitelline sys- 
tem are apparently taken over by the somatic vessels, for at such 
a period the yolk mass is still large. 
In general, I see no serious conflict between the investigations 
of Evans (’09) and the results from these observations on Am- 
phibia. In certain regions of various sectioned specimens there is 
some indication of the formation of blood vessels from early capil- 
lary networks, such, for instance, as in the body-wall of Ambly- 
stoma near the heart. Also on the surface of the yolk and in the 
body-wall of living embryos the development of later vessels is 
through the selection of certain channels of the capillary net- 
works, but the very first vessels or those developed on the yolk 
surface at an early period are more or less isolated and gradually 
anastomose with each other to form the yolk network. Many 
of these early capillaries, in both Desmognathus and Amblystoma, 
are without endothelial walls. At first many of them have no 
blood, but later there comes to be a circulation in some of the 
