416 Miscellaneous. 



cavity (-partim), but really to the digestive canal. Consequently, 

 the lining of the portion enclosed by embole represents not meso- 

 blast, but hypoblast. 



According to this, the development of the digestive canal must be 

 understood to proceed as follows : — At the oral pole of the ovum 

 there appears a depression, which is clothed by the epiblast which 

 is driven inwards. This depression, becoming deeper, pushes back 

 the subjacent layer, that is to say the hypoblast, which yields and 

 becomes eaten away after a certain time. In this way a communi- 

 cation is established between the hypoblastic cavity and the exterior, 

 by the medium of a permanent mouth. As a matter of fact the 

 archenteron does appear trilobed in front, but the lobes belong to 

 the digestive canal. Later on, the lateral lobes commence to de- 

 generate and disappear. Then the blastopore closes, and the anus 

 is formed in its vicinity. 



At the same time as the atrophy is taking place in the lateral 

 lobes of the archenteron, a delamination sets in between the epi- 

 blast and hypoblast, and a mesoblastic cavity is formed, which Avill 

 subsequently become the general body-cavity of the animal. 



In proportion as the posterior portion of the embryo increases in 

 size, the separation between the two layers increases, and there is 

 constituted posteriorly a spacious cavity, traversed by two mesen- 

 teric bands, of mesoblastic origin, which attach the digestive caniil 

 to the somatic walls. These mesenteries are finally absorbed in 

 the posterior region of the body, where the somatic cavity is single 

 in the adult. 



On the sides of the terminal intestine cellular proliferations arise 

 at an early period, whence are derived the male and female organs. 

 The latter, therefore, are not developed, as has been asserted, in the 

 cavity of the intestine, but outside it, in the space resulting from 

 the delamination of the epiblast and h]i"[)oblast, of which we have 

 spoken above. We found it impossible to determine with certainty 

 the part played by each of these two layers in the formation of the 

 genital glands, and consequently to discover whether Edouard van 

 Eeneden's theory is here confirmed. 



We have nothing to add to what has been stated as to the mode 

 of formation of a cephalic and somatic section of the general body- 

 cavity. 



AVe ascertained that the musculature, which is tolerably comjdex 

 in the cephalic region, is derived from the mesoblast of the corre- 

 sponding division. 



We were not able to study in sufficient detail the development 

 of the nervous system. Nevertheless, from the ensemble of our 

 observations upon the erabryogony of Sagltta, we suspect that this 

 typo is not so distant from llie Vertebrates as is generally sujiposod. 



S(t(jitt(i, the Ascidians, and AmpJiio.vns appear to us to constitute 

 a special group, in which we observe the appearance of the earliest 

 lineaments of the Vertebrates, and which, for this reason, we might 

 designate bv the title '' Prevertebratcs." — Comj-U^s lit'iidns, t. cxiv. 

 no. 1 (Jan. 4. 1892\ pp. 2S, 29. 



