MUSEUM OF COMrARATIVE ZOOLOGY. 117 



Larvenleil)es andert sich jedoch allmahlig das friihere Verlialtuiss zu 

 Gunsten der Entoderrnfiilhuig, die noch immer keine wahre Ilohle, sonderii 

 eiue schmale linearej rait der Hawptachse des Leihes zusammenfallende SpaUe 

 hesitzty ■• With the growth of the entodermal layer, the coelenteron 

 enUirges, and the cleavage cavity is diminished, until finally it is 

 entirely obliterated and the entoderm everywhere comes into contact 

 with the ectoderm (Plate I. Figs. 4-6, Plate XL Fig. 11). 



During the process of gastrulation, and also for a short time after its 

 completion, the thickness of the entoderm, which is much less than that 

 of the ectoderm, does not increase. Figures 5 and 6 (Plate I.) are 

 from sections of two embryos at different stages of development. 

 Figure 5 is from an embryo soon after the completion of gastrulation ; 

 Figure 6 is from an older stage. Since in each case the section is from 

 the middle of its series, it follows that a decided thickening of the ento- 

 derm takes place between the stages represented by these Figures. This 

 thickening is apparently due to an increase in the number of the cells, 

 which are soon unable to find room for themselves except by elongation. 

 The entodei'mal cells are quite different in appearance from tliose of 

 the ectoderm ; they are approximately spherical, and do not have as 

 numerous spheroidal yolk bodies as the latter. Their nuclei, however, 

 closely resemble those of the ectoderm, and usually lie in the portion of 

 the cell nearest the coelenteron. 



As is to be seen from Plate II. Fig. 7, — a section nearly perpendicular 

 to the blastoporic canal, — the blastopore in A. flavidula is very small. 

 A similar condition has been shown by Claus to exist in A. aurita, and 

 by Metschnikoff ('86, Taf. X. Fig. 14) in Nausithoe margiuata. 



The nuclei of the cells composing the wall of the blastosphere are sit- 

 uated, as has been stated, near the surface of the sphere. But at about 

 the time of the beginning of the invagination, sometimes a little earlier, 

 a few of the nuclei are found in the deeper portion of the wall. At first 

 there are only one or two such displaced nuclei to be observed in the 

 whole embryo, but as development progresses they increase in number. 

 A careful examination of sections shows that the cells to which they 

 belong do not extend, like the remaining cells of the wall, through its 

 whole thickness, but that they are wedged in as it were between the 

 bases of the ordinary cells. The latter are much elongated, and from 

 mutual pressure are prismatic, whereas the deep cells are spheroidal and 

 project in some cases into the segmentation cavity. Since these cells 

 are found at various intermediate positions between the outer and inner 

 ^ The original is not Italicized. 



