DEVELOPMENT OF CUCUMARIA. 635 



ectoderm formed a single tissue, and this is observable in the 

 later larval stages and the pentacula of G. saxicola and C. nortnani. 

 Of these species up to the third day stage it is certainly not true, 

 the ectoderm, except near the blastopore, being a definite, single- 

 layered epithelium. My preparations do not eonfii'm the same 

 author's further statement that mesenchyme originates from the 

 definitive ectoderm (except for the abov^e-mentioned cytoplasmic 

 fragments, and except in unhealthy or abnormal individuals). 



The fully-formed gastrula is very remarkable (PI. I. fig. 7). 

 The archenteron is deeply invaginated and forms a flattened, 

 thick-walled vesicle. In one lateral aspect (as made out from 

 sections J this vesicle is almost circular ; at right angles to this 

 direction it is seen edgewise, and appears dumbbell-shaped in 

 section owing to the central inflection of its flattened sides. Its 

 cavity, in other words, is a disc with a thickened periphery except 

 where this is interrupted by the blastopore. I have found this 

 stage in both the species examined. 



The details of the process by which the primary coelomic 

 pouches are formed from the archenteron I am unable to give. 

 What is certain, however, is that the water-vascular system, the 

 posterior (perivisceral) coelom, and the gut are derived, in the 

 order named, from successive regions of the archenteron, be- 

 ginning at the anterior end. At the stage shown in fig. 8 the 

 primary pouches are already present as thick-walled vesicles, still 

 in connection with the gut and with one another. The large 

 anterior pouch gives rise to the stone-canal and to the rest of the 

 water- vascular system, and may therefore be supposed to represent 

 anterior coelom plus hydrocoel. It is a flattened sac which crosses 

 the larval axis obliquely and curves back to communicate with 

 the posterior coelom. Nine sections on either side of the one 

 figured show these two sacs in the same relative positions, but 

 their connection with one another persists through only three 

 sections in all, and the posterior coelom communicates with the 

 gut in one section only — that next to the one figured. 



Apparently this state of affairs has been brought about by an 

 S-shaped bending of the whole of the archenteron at right angles 

 to its plane of flattening, coupled with a pinching-ofF of its 

 anterior three-quarters proceeding inwards from opposite edges 

 of the original disc. 



This stage I have seen in C, Hovmani only. 



There is no indication yet of the position either of the madre- 

 poric pore or of the stomodseum, and it is therefore impossible, 

 in the absence of annectant stages, to determine with certainty 

 the dorsal and ventral sides of the larva. It will be remembered 

 that in Synavta the coelom is described by Selenka as being 

 budded off dorsalwards from the archenteron, and a similar 

 orientation has been ascribed to the primary vesicle in C planci 

 by the same author, and in Holothuria Jlortdana by Edwards (4). 

 Ludwig states, without giving the grounds for his opinion : 



" Dixs Hydro- Enterocoel liegt nicht, Avie Selenka angiebt, 



44* 



