454 



EMBRYOLOGY 



Some internal developmental processes still remain for us to consider, 

 in which, however, we must confine ourselves to the fundaments of the 

 chief systems of organs. 



We saw that the body cavity of the young larva consists of two separate 

 spaces, namely the subambulacral and the visceral body cavities (Figs. 

 219 Ls and Lv and 220 Ij) and rp). In the development of the arms 

 these two spaces are continued into them and give rise to their ventral 

 and dorsal canals (Perrier). The separation of the body cavity into 

 these primitive spaces does not, however, persist long. The mesentery 

 lying between the subambulacral and visceral body cavities jjartly dis- 

 appears, and the two thus coalesce. On the other hand, a membrane, 

 which marks off a central space (Fig. 224 ax) — lying approximately in 

 the perpendicular axis of the body — from the rest of the body, makes 

 its appearance as a new structure. Another such boundary is developed 

 in the periphery of the intestine (Fig. 224 (7) ; it is called the visceral sac. 

 The part of the body cavity lying external to it is called by Ludwig the 

 circumvisceral, the inner part, on the other hand, the intervisceral, body 

 cavity 1 (Fig. 224 cl and (7). Enclosed in the latter lies the axial cavity 

 (ax) already mentioned. After the disappearance of the primary mesen- 

 teries and the development of these spaces, the ventral canal of the arm 

 coalesces with the axial, and the dorsal cavity with the circumvisceral 

 cavity (Fig. 224). 



The genital canal, which encloses the genital cavity, extends in the 

 arms between the dorsal and ventral canals (Fig. 224 g). These structures 

 also take their origin from the disc. According to Perkier, the genital 

 apparatus is established at an early period, even before the pentacrinoid 

 stage is reached. It then consists of a thickening of the splanchnic layer 

 of the visceral body cavity and occupies an axial position in the lower 

 part of the cup. It remains here for a while, and so changes as to 

 acquire a racemose appearance and, within, a cavity. After the develop- 

 ment of the arms has taken place, it splits at the tip and sends a branch 

 into each arm. This mode of origin of the genital apparatus agrees with 

 the recent description of Hamann (No. 21), according to whom there is 

 in other Echinoderms also a central part of the genital apparatus, from 

 which branches go off to the different radii. The additional branching 

 of the genital tubes to the pinnules in the Crinoids is then homologous 

 with the development of the genital sacs of other Echinoderms. 



As regards the blood-vascular system, we have already learned that 

 the chambered organ lying in the centrodorsal piece takes its origin from 

 the outer enterocoelic lamella, which penetrates into the larval stalk 

 (Peuhieb). This splits into five cords, which acquire cavities and then 



' The presence of separate spaces of the body cavity is not admitted 

 by Hamann {Histologie der Crinoiden, Jena, 1889). We have followed the 

 statements of Ludwig, and as regards embryological questions have 

 adhered to those of Perkier, 



