UROCHORDA. 



they meet and coalesce at the posterior end of the blastoderm 

 behind the nervous disc (fig. 13, cl}. The tubes themselves at 

 the same time become slightly constricted not far from their 

 hinder extremities, and so divided into a posterior region nearly 

 coterminous with the nervous system (fig. 13), and an anterior 

 region. These two regions have very different histories in the 

 subsequent development. 



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 processes are clearly of 

 the same nature as those which 

 take place in Chordata 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 ali- 

 mentary cavity becomes, by the 

 growth of the layer of cells de- 

 scribed in the last stage, a com- 

 plete 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 



FIG. 13. BLASTODERM OF PVRO- 

 SOMA SHORTLY BEFORE ITS DIVISION 

 INTO CYATHOZOOID AND ASCIDIO- 

 ZOOIDS. (After Kowalevsky.) 



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

 dostyle ; at. atrial cavity ; ng. nervous 

 groove. 



The heart and pericardial cavity are 

 seen to the left. 



