MUSEUM OF COMPARATIVE ZOOLOGY. dg 
form among other structures the calcareous rods and the suspensoria, 
filiform bodies which connect the hypoblast and epiblast. In Echinus 
miliaris and Toxopneustes, according to Selenka (op. cit. p. 46), they 
also form certain muscles of the stomach and intestine.* I cannot 
at present say whether these mesoblastic cells originate from the hypo- 
blast alone, or from the epiblast as well, in the genus Echinarachnius. 
It seemed to me that they arose from a neutral zone on the region of 
the blastopore. This zone or region, from its position at this time, is 
either epiblast or hypoblast, or both. As, however, the hypoblast is 
formed of infolded cells, which elsewhere are later epiblast, we might 
say that cells originating from this neutral zone are strictly derived 
from the epiblast. The observations of several naturalists are at vari- 
ance on this point, as far as the gastrula of other Echinoids is concerned. 
Selenka holds that in Hchinus microtuberculatus, Spherechinus granu- 
laris and Arbacia pustulosa the mesoderm cells spring from the hypo- 
blast. Other naturalists, as Greef, Metschnikoff and Bergh, derive 
them from the epiblast as well, in these and other genera. 
Ludwig, who has reviewed the different observations of the embry- 
ologists who have studied the question, concludes that in general the 
mesoderm cells arise from the hypoblast, but that ‘“‘auch aus dem Ekto- 
derm sich Zellen abschniiren und zu Mesodermzellen werden kénnen.” 
No special observations were made on the character of the contents 
of the segmentation cavity, and the space between hypoblast and epi- 
blast in the gastrula. LudwigT regards it as filled with a liquid 
through which the mesoblastic cells can move in Asterina. This seems 
more natural than to regard it with Hensen ¢ as occupied by “ Gallert- 
kern” or any fixed gelatinous structure. 
Gastrula. 
As the infolded blastoderm or hypoblast pushes its way in the form 
of a pouch into the segmentation cavity, it changes its form from a 
simple infolding to a funnel-shaped tube, the parts of which are at first 
undifferentiated. The primitive opening, blastopore, or gastral mouth, 
Pl. IV. fig. 1, gm, would seem to serve as both mouth and anus, since 
there is no other communication with the outside water. Krohn says 
* The question what structures in the Echinoderm pluteus these cells form is 
a complicated one, and has been variously answered. 
Tt Op. cit. p. 14. 
t Arch. f. Naturg. 1868. 
