UROGHORDA. 



21 



The nervous disc has during these changes become marked by a 

 median furrow (fig. 13, ng), which is soon converted into a canal 

 by the same process as in the simple Ascidians. The closure 

 of the groove commences posteriorly 

 and travels forwards. These pro- 

 cesses are clearly of the same nature 

 as those which take place in Chor- 

 data generally in the formation of 

 the central nervous system. 



In the region of the germinal 

 disc which contains the anterior part 

 of the atrial tubes, the alimentary 

 cavity becomes, by the growth of the 

 layer of cells described in the last 

 stage, a complete canal, on the outer 

 wall of which the endostyle is formed 

 as a median fold. The whole anterior 

 part of the blastoderm becomes at 

 the same time gradually constricted 

 off from the yolk. 



The fate of the anterior and 

 posterior parts of the blastoderm is 

 very different. The anterior part 

 becomes segmented into four zooids 

 or individuals, called by Huxley 

 Ascidiozooids, which give rise to a 

 fresh colony of Pyrosoma, The posterior part forms a rudimentary 

 zooid, called by Huxley Cyathozooid, which eventually atrophies. 

 These five zooids are formed by a process of embryonic fission. This 

 fission commences by the appearance of four transverse constrictions 

 in the anterior part of the blastoderm; by which the whole blastoderm 

 becomes imperfectly divided into five regions, fig. 14 A. 



The hindermost constriction (uppermost in my figure) lies just 

 in front of the pericardial cavity; and separates the Cyathozooid 

 from the four Ascidiozooids. The three other constrictions mark off 

 the four Ascidiozooids. The Cyathozooid remains for its whole 

 length attached to the blastoderm, which has now nearly enveloped 

 the yolk. It contains the whole of the nervous system {ng), which 

 is covered behind by the opening of the atrial tubes {cl). The 

 alimentary tract in the Cyathozooid forms a tube with very delicate 

 walls. The pericardial cavity is completely contained within the 

 Cyathozooid, and the heart itself {ht) has become formed by an invo- 

 lution of the walls of the cavity. 



The Ascidiozooids are now completely separated from the yolk. 

 They have individually the same structure as the undivided rudiment 

 from which they originated ; so that the organs they possess are 

 simply two atrial tubes, an alimentary tract with an endost}'le, and 

 undifferentiated mesoblast cells. 



Fig. 13. Blastoderm of Pyrosoma 

 shortly before its division into cy- 

 ATHOZOOID AND Ascidiozooids. (After 

 Kowalevsky.) 



cl. cloacal (atrial) opening; en. en- 

 dostyle; at. atrial cavity; ngf. nervous 

 groove. 



The heart and pericardial cavity are 

 seen to the left. 



