MUSEUM OF COMPARATIVE ZOOLOGY. 115 
from Apostolides’ paper would seem to indicate: “ Le jugement de M. 
Balfour repose sur de simples probabilités de ressemblance avec le type 
Holothurie, dont l’embryologie lui sert comme plan général de tous les 
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, 
Pl. 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 the truncated pole 
bulge somewhat into the cavity of the blastosphere, and from it meso- 
blastic or amceboid, spherical, and star-shaped cells, a cl, 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 Ophiophragma. 
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. Metschnikofft found it 
very difficult to observe the “ Mesoderm formation” in Ophiothrix fra- 
gilts, 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 
Echinoderms the “ Mesodermkeim” 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. Zodl., XLII. p. 664. 
