MUSEUM OF COMPARATIVE ZOOLOGY. 121 
firm wall of the ectoderm from escaping, the pressure exerted upon it by 
the enlarging entoderm is probably sufficient to cause it to be forced 
through the entodermic wall into the cwlenterie cavity. From Figure 10 
it is to be seen that one cell has already reached the gastral cavity. In 
speaking of these peculiarly situated cells I have thus far assumed that 
they are such as originally reached the cleavage cavity by an early 
ingression, where, with changed nuclear condition, but apparently with 
no further alteration, they have remained until the time of gastrula- 
tion. That this is their source is evident from the following consid- 
erations. First, the small diameter of the blastoporie canal (Plate II. 
Fig. 7), which is from the same series as Figures 9 and 10, precludes 
the assumption that they might have entered the gastrula cavity from 
without. Secondly, in their large size and general appearance they are 
unlike the cells of either ectoderm or entoderm at any time during 
gastrulation, and so could not have been derived from those sources 
during that process. Thirdly, they do correspond in size and general 
characters, except in their nuclear conditions, with the cells of the 
blastospheric wall as the latter appear at the time when ingression 
takes place. 
It is difficult to state either the cause or the purpose of this immigra- 
tion. That it is not essential to the welfare of the embryo, either by 
affording nourishment to the developing cells of the entoderm, or in 
any other way, is evident from the fact that in a large number of cases 
it does not occur. That it is not an inherited tendency, derived from a 
more primitive method of gastrulation by ingression, is probable from 
the fact that the immigrating cells do not appear to have any share 
whatever in the formation of the entoderm. On the other hand, its 
oceurrence scems to be much too frequent to be considered as acci- 
dental. 
I have stated previously (p. 119) that two very different kinds of cells 
are to be found at times in the cleavage cavity. Besides the large immi- 
grating cells already described at length, I have found in a much smaller 
number of cases very small cells (Plate I. Fig. 2), one or two in num- 
ber, that appear precisely like the deep-lying ectodermal cells already 
described. Because: of their strong resemblance to the latter, their 
exceptional occurrence, and the fact that they do not appear until after 
the beginning of the development of the deep-lying ectodermal layer, 
I incline to the opinion that they are derived from that layer, and that 
their occurrence is entirely accidental. 
At first it appeared to me surprising that two investigators could 
