458 NOTES AND MEMORANDA. 



2nd part, 1878. It will be remembered that since the appearance 

 of Haeckel's monograph on the group^ there has been a con- 

 tinuous series of papers by Metschnikoff, Oscar Schmidt, i\ E. 

 Schulze, Barrois, and Keller, giving the most contradictory 

 accounts of the formation of the diblastula. No one had 

 succeeded since Metschnikoff in observing the transition from 

 the free-swimming to the fixed condition. According to Met- 

 schnikoff, the blastula of S^con ciliatnm consists of a hemisphere 

 of long ciliate cells, and a hemisphere of large, rounded, granular 

 cells ; the ciliate cells become invaginated to form the endo- 

 derm — whilst the large granular cells form the ectoderm and 

 skeleton. The blastopore closes^ and a new osculum forms by 

 rupture. (See this Journal, vol, xv., p. 78.) Metschnikoff's 

 account seemed very improbable and for a time embryologists 

 were completely led astray by the " pseudo-gastrula/'' described 

 by 1\ E. Schulze, as the true gastrula or diblastula. (See this 

 Journal, vol. xvi, p. 65.) Curiously enough, the embryo-sponge, 

 whilst still in the brood-cavities of the parent, exhibits a tem- 

 porary concavo-convex form, in which it is the large ronnd cells 

 2i)Inch are invaghiaied, and not the ciliate cells. This invagina- 

 tion was very naturally mistaken by Schulze for the endodermal 

 invagination. The definite result of Schulze's last paper is, 

 however, fully to confirm Metschnikoft's account as to the per- 

 manent invagination. Schulze has succeeded in keeping the 

 embryos under observation whilst they fix themselves. His 

 observations show that the blastopore becomes the pole of 

 attachment. 



