ECHIN0CYAMU8 PUSILLUS. 



21 



In a full-sized Blastula, the cells not only at the vegetative pole, 

 but even those at the animal pole attain a greater length and acquire 

 an almost columnar form. The cilia at both the opposite poles are 

 very large. 



The formation of the mesenchyme, which will be treated more 

 fully in a following chapter, has taken place in the larva at a rather 

 early period. When the larva has reached an age of about fifteen hours, 

 PL II, fig. 36, cells at the vegetative pole lose their connection with 

 the surrounding cells and wander as amœboid cells into the blastocœl. 

 Presently we find that these wandering cells have arranged themselves 

 into two bilateral heaps, one on each side of the Blastula, PL II, fig. 

 33 — <5J, and that at about twenty hours after the fecundation, an extremely 

 small calcareous body arises, originated b}^ the heaps of cells. Thus, 

 the calcareous spicules begin to develop during the Blastula stage. 



Special attention ought to be paid to the differentiation of the 

 blastomeres, because it is obvious that we now have to do with three 

 different kinds of cells, those at the animal pole, those at the vegetative 

 pole and the rest of the blastomeres, and that they serve to perform 

 different functions. Thus the cylindrical cells provided with very long 

 cilia, which may be found forming a small circular thickened area at 

 the animal pole of the Blastula, remind one obviously of the apical 

 disk with its tuft of cilia common in larvas of Annelids and other everte- 

 brates. One is almost tempted to beUeve that they are larval sensory 

 cells and that their long cilia have nothing to do with the locomotion. 



Nachtrieb ') also has called our attention to a similar arrange- 

 ment in the larva of the Clypeastroid Mellita. He says: »At the pole 

 opposite to where the blastopore is formed is a small circular area in 

 which the cilia are longer, stronger, and less active than those at any 

 other point of the larva. When the Gastrula is swimming these long 

 cilia are directed forwards, now and then sway to and fro slightly, but 

 never aid in propelling or turning the larva, apparently being inactive 

 except that occasionally they seem to act as sensory cilia». This quite 

 agrees with what I have found in Echinocyamus, On the contrary 

 Fewkes^) did not pay any special attention to this apical area in the 

 larva of Echinarachnius. 





1) Preliminary Notes on the Echinoderms of Beaufort. 1885 p. 66. 



2) Preliminary Observations on the Development of Ophiopholis and Echina- 

 rachnius. 1886. p. 128. ■ 



