MUSEUM OF COMPARATIVE ZOOLOGY. 119 
The subsequent history of these cells, as shown by the comparison of 
specimens of succeeding stages of development is both interesting and 
peculiar. I imagine that it is such cells as these to which Claus (90, 
p. 3) refers when he says: “Ich habe den vereinzelt eingetretenen zwei 
bis drei Zellen, weil sie nicht regelmässig in jeder Blastula sich ablösen, 
der am vegetativen Pole einwuchernden Zellenmasse gegenüber keine 
weitere Bedeutung beigemessen, so dasse ich dieselben zwar auf einer 
Abbildung darstellte, im Texte aber nicht besonders erwähnte, und bin 
auch jetzt noch der Ansicht, dass diese auffallend kleinen Zellen wieder 
rückgebildet werden und überhaupt nicht zur Bildung des Entoderms 
beitragen.” In my judgment, a part of the difference of opinion be- 
tween Goette and Claus is due to the fact that there are two kinds of 
cells which find their way into the cleavage cavity. These are the large 
cells described by Goette as beginning to be formed at an early stage of 
the blastula, and much smaller cells, of which I shall have more to say 
hereafter, that make their appearance only at later stages of develop- 
ment. Claus seems to have seen “very small cells,” and to have 
assumed that they were equivalent to the large cells figured by Goette. 
I am unable to say with ‘certainty that the cells seen by Claus are the 
equivalents of those figured by Goette, but Claus assumes that they are, 
and I have the more reason to believe it because the large cells are of 
more frequent occurrence than the small ones. But if this be so, I do 
not understand how Claus could speak of them as “diese auffallend 
kleinen Zellen.” But however that may be, I have reason to believe 
that the supposition of Claus, that they ultimately degenerate, is 
correct. 
Soon after the ingression of a cell its nucleus undergoes changes 
which result in its disappearance as such, for instead of a nucleus there 
can be seen only one or more small, isolated, deeply stained particles, 
which I judge to be scattered portions of the nuclear chromatine 
(Plate II. Figs. 8 and 10). Even these are often wanting. I have 
said that this nuclear change follows soon after the ingression of the 
cell, because out of the numerous instances in which these cells have 
been present there is not one in which the nucleus retains its original 
condition after the cells in the wall of the blastula have given evidence, 
by their diminished size, that they have undergone division since the 
ingression took place. This conclusion is in part based on the assump- 
tion that at the time of ingression the ingressing cells are of about the 
same size as those which remain in the wall of the blastula. The in- 
gressing cells sometimes persist, without any further apparent changes 
