26 Hjalmar Théel, 



Judging from the figures given by Selenka, a differentiation of the cells 

 at the animal pole exists in some forms, especially in Echinocardium 

 cordatum, while there is no trace of such a differentiation in other forms, 

 as for instance in Strongylocentrotus lividus etc. In Dorocidaris pa- 

 pillata ') the larva in this state of the development is oval v?ith a tuft 

 of long cilia at the animal pole. In Strongylocentrotus dröbachiensis 

 the Gastrula also acquires an oval shape, according to Agassiz ^), and 

 the cells at its animal pole seem to be considerably higher. 



On the whole, the present literature shows that the Gastrulse in 

 most Echinoids are spherical or slightly oval and that they never or only 

 exceptionally reach the length which characterizes the Gastrula of Echi- 

 nocyamus. Concerning the thickened area at the apical pole, there is 

 scarcely any information available, but the figures drawn by the investi- 

 gators show that such an area is present in several forms. As far as 

 I know, only Nachtrieb has clearly pointed out the presence of this 

 »sensory» disk in some Echinoids. 



VI. 

 Formation of the Mesenchyme. 



Considering that several different opinions have been promulgated 

 as to the origin of the mesenchyme in the Echinoderms and that the 

 point still remains undecided, I paid special attention to it, and the 

 more so as the question has gained a certain interest. I may first be 

 allowed to give a short account of these different views. 



Hensen ^) was the first to observe in the larva of a starfish, that 

 the mesenchyme cells, which were met with in the segmentation cavity, 

 took their origin from the vegetative pole of the larva or rather from 

 that part which was in a state of invagination and that they gradually 

 lost their connection with the other cells. In the year 1869 this state- 



1) Recherches sur le Dorocidaris papillata et quelques autres Echinides de la 

 Méditerranée. 1887. p. 343—344. 



2) Revision of the Echini. IV. 1874. p. 711—712. 



8) Ueber eine Brachiolaria des Kieler Hafens. Archiv f. Naturg. XXIX. 

 Berlin 1863. p. 242. 



