MUSEUM OF COMPARATIVE ZOOLOGY. 127 



form among other structures the calcareous rods and the euspensoria, 

 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 iu Echinus microtuherculatus, Sphcerechinns granvr 

 laris and Arbacia pu^ttdosa the mesoderm cells spring from the hypo- 

 blast. Other naturalists, as Greef, MetschnikofT and Bergh, derive 

 them from the epiblast as well, in these and other genera. 



Ludwig, who has reviewed the diflferent 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 Zelleu abschniiren und zu Mesodermzellen werden konnen." 



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. Ludwig f 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 X 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, 

 PI. 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 



* Tlie question what structures in the Echinoderm pluteus these cells form is 

 a complicated one, and has been variously answered. 

 t Op. cit. p. 14. 

 t Arch./. Xaturg. 1863. 



