115 



ing of the amniotic cavity, the median position of which, so diflerent 

 from the normal position of the amniotic opening in Echinoid larva', as 

 emphasized Ijy Miss Medes (Oj). cil. p. I'M), is thereby naturally ex- 

 plained. As Miss Medes had only one single stage for sectioning, the 

 natural explanation of the median position of the amniotic opening could 

 not possibly be found out by her, and the statement that there was "no 

 indication of the formation of a mouth" was an equally unavoidable con- 

 sequence of the lacking of the younger developmental stages. This also 

 accounts for the fad that Miss Medes thinks the jjosilion of the amniotic 

 opening to be ventral, "so that the dorsal surface of the pluteus forms 

 the aboral surface of the adult" (()]). cit. p. 134). .\s shown definitely by 

 the course of the development it is, on the contrary, the dorsal surface 

 of the Pluteus which becomes the oral surface of the aflult. 

 the ventral surface of the larva becoming the aboral side of the sea-urchin. 



Contemporaneously with the formation of the amniotic prolongation 

 from the pharynx the blastoporus, or, as it might now be termed, the anal 

 opening wanders upwards along the ventral side (PI. XV, Figs. 20 — 21), 

 where it may renuiin distinct for a little while yet; the last traces of it 

 have disappeared at the age of about 2.^ hours. The posterior part of the 

 entoderm is seen to lie in a transverse position on the ventral side, some- 

 what transplaced towards the left side. Its lumen remains distinct, and it 

 appears to be somewhat curved (PI. XVI, Figs. 4, 8). This part of the 

 entoderm, which develops directly into the intestine of the sea-urchin, 

 soon separates completely from the larger, originally anterior, part of the 

 entoderm, from which the hydrocoel and enterocoel develop. 



The lumen of the hydrocoel begins to difi'erentiate at the age of 23 — 24 

 hours; but 1 cannot enter on a detailed description of the complicate pro- 

 cesses of the dilTerentiation of the hydrocoel and the enterocoel. I would 

 only emphasize that no subdivision of the enterocoel lakes place. 

 The part to the aboral side of the intestine may have the appearance of 

 being separate; a careful examination of the series of sections shows, how- 

 ever, definitely that it is in connection with the rest of the body cavity, 

 there being in fact only one, common space. Accordingly 1 cannot agree 

 with Miss Medes in designating the diflerent parts of the body cavity as 

 anterior or jjosterior enterocoel. We have here only a general body cavity, 

 and a special homology between its dilTerenl parts and the anterior and 

 posterior enterocoel of typical l^chinoid larv;e cannot be carried through. 



.\ very important jjoinl is the formation of the hydropore. Miss Medes 

 in her figure 22 represents a double series of nuclei forming something like 

 a canal, which appears to ()])en on the dorsal side. This is taken to be the 

 pore canal. I must doubt the correctness of this view. Such series of nuclei 



15* 



