122 BULLETIN OF TUE 



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

 "JO) and (ioette ('B7), concerning the method of gastrulation in the 

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

 Hcenis loHH strange. The results obtained from my first sections led me 

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

 I he case of A. finvidula. lietter staining, tiiinner sections, and more 

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

 gastruhition in tliis species is much more in accord with the description 

 given by Chius, and tiiat the process really is one of invagination. 



(Jertain considerations weaken my confidence in the position defended 

 by (ioette. A ooinparison of his Figures G-9 ('87, Taf. I.) with some 

 of my tliicker sections, or with those wliich were made when the gastrula 

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

 it a[)pear to me [>rolKiblo that his results are based upon similar inade- 

 quate sections. In Figure 8 (Plate II.) tliere 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 fjoette ('87, Taf. I.), nuclei are 

 figured in nearly all the celh. I believe this 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 enfodormal cells arc usually situated in the portion of the cell near- 

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

 cinliryo 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 Figin-o 3 (Plate I.). The intervening section (not 

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

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

 like ]<'igure 12, or such as are a little oblique to tlie chief axis of the 

 embryo h;ive the appearance of containing immigrating cells. Such 

 sections also fixhibit tlu; flattening in the region of the shorter cells to 

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

 "Sehon wilhreud dor Gastrul;itiou zeigt sich eiue Stelle des Keims im 

 PcnMch seiner kiirzeren Zellen etwas al)geplattet." 



Additional considerations increase the prf)bal)ility of the correctness 

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

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

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



