122 BULLETIN OF THE 



reach such different conclusions as those published by Clans ('83 and 

 '90) and Goette ('87), concerning the method of gastrulation in the 

 same animal, A. aurita. Since studying this process in A. flavidula, it 

 seems less strange. The results obtained from my first sections led me 

 to think that the conclusions reached by Goette would be confirmed in 

 the case of A. flavidula. Better staining, thinner sections, and more 

 accurate orientation have made it certain, however, that the method of 

 gastrulation in this species is much more in accord with the description 

 given by Glaus, and that the process really is one of invagination. 



Certain considerations weaken my confidence in the position defended 

 by Goette. A comparison of his Figures 6-9 ('87, Taf. I.) with some 

 of my thicker sections, or with those which were made when the gastrula 

 was so oriented as not to be cut parallel to the blastoporic canal, makes 

 it appear to me probable that his results are based upon similar inade- 

 quate sections. In Figure 8 (Plate II.) there are only about one half 

 as many nuclei visible as there are cells, the nuclei of a portion of the 

 cells being contained in adjacent sections. In figures of corresponding 

 stages of A. aurita as represented by Goette ('87, Taf. I.), nuclei are 

 figured in nearly all the cells. I believe tliis to be evidence that his 

 figures were drawn from thick sections. The blastopore, because of its 

 very small diameter, is quite easily overlooked in thick sections, and 

 especially if the plane of sectioning is somewhat oblique to the longitu- 

 dinal axis of the blastopore. Since, as previously stated, the nuclei of 

 the entodermal cells are usually situated in the portion of the cell near- 

 est the ccelenteron, it is easy to find in thick sections of an invaginating 

 embryo conditions like those represented by Goette in his Figures 6-8. 

 My Figure 12 (Plate II.) reproduces a section of the same series as 

 that represented in Figure 3 (Plate I.). The intervening section (not 

 figured) is quite similar to Goette's Figure 8. An examination of the 

 cells bordering the blastoporic canal in Figure 3 will show how sections 

 like Figure 12, or sncli as are a little oblique to the chief axis of the 

 embryo have the appearance of containing immigrating cells. Such 

 sections also exhibit the flattening in the region of the shorter cells to 

 which Goette ('87, p. 4) has called attention in the following words : 

 "Schon wahrend dor Gastrulation zeigt sich eine Stelle des Keims im 

 Bereich seiner kiirzeren Zollen etwas abgeplattet." 



Additional considerations increase the probability of the correctness 

 of the view which I have advanced to explain Goette's error. With 

 advancing stages of development, I have found an increase in the num- 

 ber of the cells composing the ectodermic wall. This is undoubtedly 



