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 Glaus ('90, 

 p. 3) refers when he says : " Ich habe den vereinzelt eingetretenen zwei 

 bis drei Zellen, weil sie nicht regelmassig in jeder Blastula sich ablcisen, 

 der am vegetativen Pole einwuchernden Zellenmasse gegenuber keine 

 weitere Bedeutung beigemessen, so dasse ich dieselben zwar auf einer 

 Abbilduug darstellte, im Texte aber nicht besouders erwiihnte, und bin 

 auch jetzt noch der Ansicht, dass diese auffallend kleinen Zellen wieder 

 riickgebildet werden und uberhaupt nicht zur Bildung des Entoderms 

 beitragen." In my judgment, apart 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 occuiTence 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 



