1882 .] 
448 
[Kingsley. 
that of many Molluscs. In the eggs of an undetermined ISTudi- 
branch which I have studied the steps were almost exactly 
similar, and other observers present similar results from the eggs 
of other Gasterojmda. 
These blastomeres continue to divide, the next step being the 
production of sixteen micromeres, while there are still only four 
macromeres (fig. 5), the smaller ones dividing much more rapidly 
than the larger, and thus the smaller gradually extend over the 
larger, enveloping them, and forming a modified gastrula by epiboly 
and not by normal invagination. Figs. 5, 6, and 7 show several 
steps in the process. In fig. 6 the epiblast has covered about 
half of the hypoblast, and in fig. 7 the gastrula stage is reached, 
the place (b) where the hypoblast last was seen being the blasto- 
pore. I have nothing to offer on the formation of the archenteron, 
but my observations showed that in this species the blastopore 
becomes entirely closed. 
After the closing of the blastopore, the epiblast continues a 
rapid subdivision and its cells by mutual pressure acquire a poly- 
gonal outline, and the surface of the embryo becomes quite 
smooth. Soon a portion of the epiblast cells begins to divide much 
more rapidly than the rest and at the same time they become 
sharply marked off from the rest of the surface by a groove and 
a ridge which in their mode of origin, their position, and their 
purpose, seem to be the liomologue of the tail fold of the other 
vertebrates. The groove gradually makes its way in under the 
smaller celled portion of the epiblast and at the same time the 
now outlined tail begins to increase in length. This stage is 
seen in fig. 8 which (with the exception of the “ test cells”) is, cell 
for cell, an exact camera sketch. Here will be seen the modified 
characters of the cells of the tail which are transversely elongated 
and much smaller than are those of the rest of the surface. A 
profile of the embryo at this time showed that as yet the tail does 
not stand out from the body but is imbedded in it. At this time 
no trace can be seen of any internal structure, but with an increase 
in length the tail becomes more transparent and one can obtain a 
view of some of the internal features. Fig. 9 shows an embryo 
which was artificially liberated from its envelope. We here see 
the anterior end as opaque as ever and can only make out the 
