486 



INVERTEBRATA 



CHAP. 



Segmentation takes place with great rapidity, and in four or five 

 hours a small spherical blastula, consisting of relatively few cells, is 

 formed. Then primary mesenchyme (p.mes, Fig. 373, A) becomes 

 budded off from one pole, and occupies a large portion of the 

 blastocoele. The side of the blastula from which this is budded off 



a.vac 



p.mes 



-p.mes 



cce 



al 



p.vac 



FIG. 374. Longitudinal frontal sections of early larvae of Ophiothrix fray His. 



(Original.) 



al, alimentary canal ; arch, archenteron ; a.vac, anterior vacuolated crest ; coe, rudiment of coelom ; 

 p.l.a, postero-lateral arm; p.mes, primary mesenchyme; p.vac, posterior vacuolated crest; s.mes, 

 secondary mesenchyme. 



is of course the vegetative pole. The opposite pole of the blastula 

 develops into a great conical protuberance, which we may term the 

 anterior vacuolated crest. This arises (a.vac, Fig. 373, B) as a 

 consequence of the growth in height of the cells which form the 

 blastula wall in this region. They become changed into elongated 

 pillars, and develop clear vacuoles in their interior (a.vac, Fig. 374, A), 



