68 



tion in the egg. On the convex side of the bocly-j^ortion the 

 fusiform nervous cavity lies immediately under the upper 

 skin, surrounded by a wall of its own ; beneath this lies the 

 alimentary canal, distinguishable by the reddish colouring of 

 its cells, at present without an opening. The chorda, con- 

 sisting of a double row of square cells surrounded by the as 

 yet round muscle cells, reaches a little into the body-portion, 

 so that its lengthened axis would penetrate between the 

 nervous cavity and the alimentary canal. It is impossible to 

 think of a more beautiful model of a vertebrate embryo. On 

 the convex side, above the axis, we have the nerve tube; on 

 the concave side under the axis, the visceral tube; the contrast 

 of " dorsal " and " ventral " is so clearly represented that I 

 can scarcely be said to exaggerate when I say that the sight of 

 it must Avork overpoweringly on those who enter upon the 

 examination doubtingly. Without giving you in detail the 

 progressive history of development in such a hasty letter as 

 this, I will only add a little more concerning the nervous 

 system. I differ herein somewhat from Kowalevsky, although 

 in a sense which strengthens his assertions. In my Tunicate 

 the nerve-tube docs not form itself around the spherical 

 bubble Avhich contains the rising tufts which have the peculiar 

 tactile apparatus which he describes and figures. What takes 

 place is very different from what he describes in Phal. mamniil- 

 lata,foY from the bubble towards the tail, a thickish nerve-cord 

 stretches forward which contains a fine central canal which 

 opens into the cavity of the bubble. This cord extends its 

 posterior end between the muscles of the tail, so that its out- 

 line is there lost. The foremost end of the Chorda is, there- 

 fore, underneath the central nerve-system in realih/, and not 

 only when lengthened ideally. A vesicular front part and a 

 cord-like hind part of this nerve-system are distinguishable. 



So, too, one may observe the degrees of jjrogressivo deve- 

 lojoment of the escaped free-moving larva. When this period 

 is reached both openings of the alimentary canal have 

 been formed by the outer skin rapidly increasing inwards 

 (in a conical form) until it becomes imperceptibly connected 

 with the alimentary wall ; a canal then appears in the axis 

 of the newly-grown part. The mantle still closes both ojoen- 

 ings for some time. With the exception of the dorsal side, 

 where the nervous system lies between the skin and the 

 alimentary canal, the outer skin is separated entirely from 

 the intestinal Avail, so that a roomy body cavity is formed which 

 contains a continually increasing number of small round cells. 



Here the highest point of development in this direc- 

 tion is reached, and it is noAv that the second phase begins, 



