A MONOGRAPH OF THE EXISTING CRINOIDS 305 



regularly at the upper end of the invagination. Probably, according to Mortensen, 

 the formation of the mesoderm cells starts again from these cells, but at one tune 

 all the formerly loose cells have joined the entodermal invagination. 



The formation of the entoderm thus differs considerably from that of Antedon 

 in which, according to Seeliger, no such wandering of free cells into the blastocoel 

 takes place before the invagination. However, it must be mentioned that in one case 

 Mortensen found the invagination starting before any loose cells had wandered into 

 the segmentation cavity. In this case the formation of the entoderm thus proceeds 

 as in Antedon. Mortensen remarked that it is rather startling to find that there can 

 be so great variation within the same species hi so important a process as the formation 

 of the entoderm. . 



The ectoderm cells in the oral half of the embryo have their inner ends turned 

 upward, making a very characteristic arrangement. It looks as if they were pushed 

 upward by the iuvagination. The little space left by the archenteron, together with 

 the considerable elongation of the ectoderm cells at this stage, accounts for this 

 peculiar feature. 



In the lower end of the archenteron, near the blastopore, the cells are quite low; 

 in the upper wider part they are high and cylindrical. The cavity of the archenteron 

 is very narrow and makes a characteristic curve in the upper part. The blastopore 

 is a small round opening, not an elongated slit as in Antedon. 



The gastrula is fully formed about five hours after fertilization. In Antedon this 

 stage is reached, according to Seeliger, about sixteen hours after fertilization, but 

 according to Barrois and Bury it is not reached until twenty-four hours after fertiliza- 

 tion, this discrepancy being evidently due to the fact that Seeliger worked on Antedon 

 adriatica, while Barrois and Bury worked on A. mediterranea. 



In some cases Mortensen found the whole space within the egg membrane filled, 

 instead of by an embryo, with a uniform mass of minute spherules, besides a couple of 

 deeply staining protoplasm masses, looking like a pair of cleavage cells, but containing 

 no nuclei. This would appear to be some kind of parasitic organism. Mortensen said 

 that it had seemed to him worth while to call attention to this, although he was unable 

 to say more definitely what it was. 



When the gastrula stage is reached the embryo begins to rotate within the egg 

 membrane, being covered with a uniform ciliation. About six hours after fertilization 

 the rupture of the egg membrane takes place, and the embryo swims out. Only a small 

 opening is formed in the membrane, through which the embryo must squeeze itself 

 out. The empty membrane may be found on the bottom of the jar undisturbed except 

 for the hole through which the embryo crept out. 



Just after its liberation the embryo is slightly pear-shaped, being a little pointed 

 at the apical end and a little truncated at the oral end. There is thus no difficulty in 

 seeing directly that the position of the blastopore is at the posterior end of the larva. 

 Further, this shape of the embryo facilitates the orientation in sectioning, the longi- 

 tudinal axis being always distinct from the moment the embryo is liberated, while the 

 spherical embryo of Antedon cannot be oriented with certainty in sectioning until the 

 vibratile bands have been formed, the situation of the archenteron nearer the posterior 

 end and the more numerous mesenchyme cells in the anterior end affording the only 

 means by which to identify the longitudinal axis. 



