MUSEUM OF COMPAEATIVE 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 coelenteric 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 tlie time of gastrula- 

 tion. That this is their source is evident from the following consid- 

 erations. First, the small diameter of the blastoporic 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 ftict 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 shai-e 

 whatever in the formation of the entoderm. On the other hand, its 

 occurrence seems 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 ectodei'mal layer, 

 I incline to the opinion that they are derived from that layer, and that 

 their occm'rence is entirely accidental. 



At first it appeared to me surprising that two investigators could 



