534 



COMPARATIVE ANATOMY 



CHAP. 



After the process of invagination has begun, the formation of mesenchyme also 

 commences, proceeding from the blind end of the archenteron, which here is bi- 

 laminar. The cells of the layer which is turned to the segmentation cavity wander 

 into that cavity, i.e. into the gelatinous nucleus which fills it, and become mesen- 

 chyme cells (Fig. 443). The formation of mesenchyme proceeds actively during 

 the whole process of invagination along the whole archenteron, but chiefly at its 

 base. Here, indeed, the formation of mesenchyme is observed long after important 

 processes of separation and differentiation have been accomplished in other parts 

 of the archenteron. 



The formation of mesenchyme takes place here more actively than in any other 

 Echinoderm in which it has been observed, so that the large segmentation cavity 

 soon appears to be crowded with mesenchyme cells. 



The ectoderm becomes covered with cilia. 



The blastopore closes completely in the course of the second stage of develop- 



FIG. 443. A, Horizontal longitudinal section through an embryo (gastrula) of Antedon 

 twenty-six hours old ; B, the same of one forty-eight hours old, in which the archenteron, which 

 has become constricted off, is divided into two sections (after Seeliger). 1, Ectoderm ; 2, mesen- 

 chyme cells ; 3, place of formation of the mesenchyme cells at the base of the archenteron ; 4, 

 blastopore ; 5, endoderm ; 6, archenteric cavity ; 7, mesentero-hydrocoel vesicle ; 8, enterocoel 

 vesicle. 



ment. The archenteron then lies as a closed vesicle in the posterior region of 

 the segmentation cavity. 



An important process soon takes place. The archenteric vesicle or archenteron 

 becomes constricted by a circular furrow (Fig. 443 B). This constriction leads to 

 a complete division of the archenteron into an anterior and a posterior vesicle. 

 The anterior is someAvhat larger than the posterior ; the formation of the mesen- 

 chyme continues actively on its wall (Fig. 444). 



From the anterior vesicle are derived the intestine and the hydrocoel ; from the 

 posterior the ccelom with the chambered sinus, etc. We here have a remarkable 

 difference between Antedon and other Echinoderms, in which latter, as above 

 described, the anterior blind end of the archenteron always yields the coelom. 



The anterior vesicle is in close proximity to the ectoderm, on the ventral side. 



The posterior vesicle becomes a transversely placed tube, whereas the anterior 

 is produced into a horn, both dorsally and ventrally. These two horns clasp the 

 posterior vesicle from its anterior side. The larva is now distinctly bilaterally 

 symmetrical (Fig. 444). 



The next changes to occur are the following : 



