VIII 



ECHINODEEMA TA ONTOGENY 



525 



oidca, and commences either in the blastula stage or not until the gastrula stage. 

 In the former case, that part of the blastoderm which becomes invaginatecl to pro- 

 duce the archenteron yields the mesenchyme, and, after invagination has taken 

 place, continues to produce it. In the second case, also, the cndoderm is the place 

 of formation of the mesenchyme cells, which wander into the blastoccel. Such 

 observations, however, seem to point to the fact that, although most mesenchyme 

 cells arise from the endoderm, the ectoderm also takes part in their formation. 



In the older gastrula stage of Aster las vulgaris, the ectoderm seems to be 

 thickened at the (aboral) pole opposite to the blastopore. This may be the rudi- 

 ment of the neural plate. 



As a further illustration of the development of the Asteroidea, we shall utilise 

 the observations made on Asterina gibbosa, in which form, however, a typical 



ant 



post 



FIG. 424. Asterina gibbosa, 

 gastrula four days old ; ap- 

 proximately horizontal longi- 

 tudinal section, from tlie ventral 

 side (after Ludwig). ant, An- 

 terior ; post, posterior ; dex, right ; 

 sin, left ; 1, segmentation cavity ; 

 2 and 3, right and left coelomic 

 outgrowths of the archenteron ; 

 4, blastopore. 



yio.-'f 



Km. 425. Asterina gib- 

 bosa, larva at the end of the 

 fourth day, horizontal longi- 

 tudinal section seen from the 

 ventral side (after Ludwig). 

 The enterocoel outgrowth.-, 

 have grown in length. 2, right 

 enteroccel outgrowth ; 3, left 

 or hydro - enterocoel (.nit- 

 growth ; 5, intestine; 6, an-' 

 teri.>r unpaired coelom. The 

 crelom is still in open com- 

 munication with the intestine. 



FIG. 42ti. Asterina gib- 

 bosa, larva at the com- 

 mencement of the fifth 

 day, horizontal longitudi- 

 nal section (after Ludwig). 

 The enteroccel has become 

 constricted off from the 

 intestine. Lettering as 

 before. 



Bipinnaria larva does not attain development. In the course of the description 

 observations made on other Asteroids will be referred to. 



In the ovoid gastrula of Asterina, the blastopore does not lie altogether at the 

 posterior pole, but is shifted somewhat on one side, which in the further develop- 

 ment of the animal becomes distinguishable as the ventral side. Two sections can be 

 made out in the archenteron, a short cylindrical commencement (posterior section), 

 and a vesicular blind terminal part (anterior section). This description applies to 

 the gastrula on the second day. 



Third day. The rudiment of the hydro- enteroccel vesicle. The anterior 

 vesicular section of the archenteron, which represents the rudiment of the hydro- 

 enteroccel vesicle, bulges out posteriorly on each side, while its wall becomes 

 thinner (Fig. 424). The two bulgings grow out longitudinally backwards, at the 

 sides of the posterior part of the archenteron, and become the two hydro-enteroccel 



