410 E. W. MACBRIDE. 
structures placed at the sides of a long slit-like blastopore, 
seems to me to be only one of those numerous distortions of 
development which yolk produces. Starting from a blasto- 
sphere with a differentiated endodermic pole, let us suppose 
that those cells which are destined in the gastrula to form the 
ventral wall of the gut become swollen up with yolky inclu- 
sions. Then the process of invagination will be asymmetrical ; 
it will be modified to a growing of the cells which are to 
form the dorsal gut-wall over the yolky cells like a lip. A 
moment’s thought will convince anyone that this cannot take 
place on a spherical surface without a simultaneous coales- 
cence of the cells destined to form the right side with those 
destined to form the left. 
The position of the notochord with reference to the dorsal 
vessel presents at first sight a serious obstacle to homologising 
it with the notochord of Vertebrates. A little reflection will 
convince us, however, that (1) when the Vertebrate notochord 
was not as yet fully separated from the gut, it could not 
possibly have been, as it is now, dorsal to the aorta; and that 
(2) since, as Professor Spengel points out, blood-vessels are 
merely blastoccelic spaces, there was very probably at that 
period of its evolution a blood-vessel between the notochord 
and dorsal ectoderm exactly where we find one in Balanoglossus. 
As the notochord became separated from the gut it would press 
on and obliterate the vessel above it. This has actually happened 
at one point in Balanoglossus, the heart and dorsal vessel being 
joined in the proboscis neck by only an exceedingly narrow chink. 
As the dorsal vessel disappeared the ventral vessel would gain 
in importance, and so constitute the central contractile organ 
or heart. The chief point which Spengel adduces in favour of 
his theory of Annelid relationship is the structure of the blood 
system, and especially the direction of circulation. This latter, 
according to an old observation of Kowalevsky’s, which, as far 
as I can gather, Professor Spengel has not confirmed, is for- 
wards in the dorsal vessel, and backwards in the ventral, thus 
agreeing with the Annelid arrangement as against that found 
in Vertebrates. ‘This fact Professor Spengel considers as 
