26 PYROSOMA. 



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 Cyatho- 

 zooid 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 involution of the walls of the 

 cavity. 



The Ascidiozooids are now completely separated from the 

 yolk. They have individually the same structure as the un- 

 divided rudiment from which they originated ; so that the 

 organs they possess are simply two atrial tubes, an alimentary 

 tract with an endostyle, and undifferentiated mesoblast cells. 



In the following stages the Ascidiozooids grow with great 

 rapidity. They soon cease to lie in a straight line, and eventu- 

 ally form a ring round the Cyathozooid and attached yolk 

 sack. 



While these changes are being accomplished in the external 

 form of the colony, both the Cyathozooids and the Ascidiozooids 

 progress considerably in development. In the Cyathozooid the 

 atrial spaces gradually atrophy, with the exception of the ex- 

 ternal opening, which becomes larger and more conspicuous. 

 The heart at the same time comes into full activity and drives 

 the blood through the whole colony. The yolk becomes more 

 and more enveloped by the Cyathozooid, and is rapidly ab- 

 sorbed ; while the nutriment derived from it is transported to 

 the Ascidiozooids by means of the vascular connection. The 

 nervous system retains its previous condition ; and round the 

 Cyathozooid is formed the test into which cells migrate, and 

 arrange themselves in very conspicuous hexagonal areas. The 

 delicate alimentary tract of the Cyathozooid is still continuous 

 with that of the first Ascidiozooid. After the Cyathozooid has 

 reached the development just described it commences to atrophy. 



