MUSEUM OF COMPARATIVE ZOOLOGY. 115 



from Apostolides' paper would seem to indicate : ** Le jugement de M. 

 Balfour repose sur de simples probabilites de ressemblance avec le type 

 Holothurie, dont I'embryologie lui sert comme plan general de tons leg 

 Echinodermes." Since we know that the formation of a gastrula has 

 been observed also in starfishes and sea-urchins, it would have at least 

 been more just to Balfour to have inserted these types after that of the 

 Holothurians in the above quotation. 



It was noticed at the close of the first day that the thickened blasto- 

 derm begins to fold inward at one pole, and at the same time that the 

 blastoderm at that point becomes more densely pigmented. The larva, 

 PI. I. fig. 13, is now pear-shaped, slightly flattened on one side and trun- 

 cated at the pigmented pole. The flattening on one side is the first 

 indications of the ventral surface, and one of the first expressions of a 

 bilateral symmetry which later becomes so well marked by the growth of 

 mesoblastic cells. The internal surface of the cells at tlie truncated pole 

 bulge somewhat into the cavity of the blastosphere, and from it meso- 

 blastic or amoeboid, spherical, and star-shaped cells, ad, begin to bud. 

 These cells form in two lateral* clusters, PL I. fig. 14, and indicate at 

 once the position of the infolded archenteron. They are the beginning 

 of a middle layer, and from them many important structures form. The 

 least diameter of the larva is .11 mm. ; the greatest .13 mm. 



The same irregular triangular form, and the clustering of pigment 

 about the blastopore seems to be found in the gastrula of Ophiophragraa. 

 It is the presence of this pigment on each side of the gastrula mouth 

 which has been of assistance in the identification of the lateral arms, /, in 

 later stages as compared with the blastopore. A clustering of pigmented 

 cells at the lower extremity of the stomach has rendered it extremely 

 difficult for me to study the changes which go on in the formation of the 

 water tubes and other structures in this region of the embryo. The 

 walls of the stomach are yellow and green. Metschnikofi't found it 

 very difficult to observe the " Mesoderm formation " in Ophiothrix fra- 

 gilis, which he was able to artificially fertilize. 



It is supposed that our embryo can be compared with that of Am- 



* There is already a considerable literature on the question of whether in 

 Echinuderms the " Mesodermkeiin " or " Mesoderm cells " arise in a bilaterally 

 symmetrical manner as regards a " spaltartige Rinne " of the gastrula, by which 

 the symmetry, is early indicated. Selenka and others hold that they do; Metsch- 

 nikoff, that they do not. My observations show such a symmetry in the mesoblas- 

 tic cells of Ophiopholis. 



t Zeit.f. Wiss. Zool., XLII. p. 664. 



