1891.] The Evolution of the Circulatory Organs. 243 
considerably from that of the ventricle; the auricle retaining the 
thin and membranous walls which it at first possessed, while the 
ventricle has become stronger and thicker, both its exterior and 
interior surfaces being marked by the interlacement of ‘muscular 
fibres, as in the higher Mollusca. About the 65th hour the grade 
of development of the heart may be regarded as corresponding 
with that of the fish, the auricle and ventricle being perfectly dis- 
tinct ; but their cavities are as yet quite single. During the fourth 
day the cavities of the heart begin to be divided, for the separa- 
tion of the right and left auricles and ventricles. About the 80th 
hour the commencement of the division of the auricle is indicated, 
externally, by the appearance of a dark line on the upper part ot 
its wall; and this, after a few hours, is perceived to be due to a 
contraction, which, increasing downwards across the cavity, divides 
it into two nearly spherical sacs. The division of the ventricle 
commences some time before that of the auricle, and is effected by 
a sort of duplicature of its walls. At last, however, the division 
is complete, and the interventricular septum becomes continuous 
with the interauricular, so that the heart may be regarded as com- 
pletely a double organ. The progressive stages presented in the 
development of this septum are evidently analogous to its per- 
manent comann in ee various t peier s IE In the heart 
of mammals (embr e place, but more slowly. 
Soon after the TRR of the ventsiciés bai to be formed in 
the interior, a corresponding notch appears on the exterior, which, 
as it gradually deepens, renders the apex of the heart double. 
“ This notch between the right and left ventricles continues to 
become deeper until about the eighth week in the human embryo, 
when the two ventricles are quite separated from one another, 
except at their bases ; this fact is very interesting from its rela- 
tion with the similar permanent form preaented by the heart of 
Dugong. 
“ At this period the internal septum is still imperfect, so that the 
ventricular cavities communicate with each other, as in the chick, 
on the fourth day. After the eighth week, however, the septum 
is complete, so that the cavities are entirely insulated; whilst at 
the same time their external walls become more connected towards 
