PYROSOMA. 



region. At one end of the blastoderm, which for convenience 

 sake may be spoken of as the posterior end, a disc of epiblast 

 appears, which is the first rudiment of the nervous system, and 

 on each side of the middle of the blastoderm there arises an epi- 

 blastic involution. The epiblastic involutions give rise to the 

 atrial cavity. 



These involutions rapidly grow in length; and soon form 

 longish tubes, opening at the surface by pores situated not far 

 from the posterior end of the blastoderm. 



The blastoderm at this stage, as seen on the surface of the 

 yolk, is shewn in fig. 12 A. It is somewhat broader than long. 

 The nervous system 

 is shewn at n, and at 

 points to an atrial 

 tube. A transverse 

 section, through about 

 the middle of this 

 blastoderm, is repre- 

 sented in fig. 12 B. 

 The epiblast is seen 

 above. On each side 

 is the section of an 

 atrial tube (at}. Below 

 is the hypoblast which 

 is separated from the 

 yolk especially in the 

 middle line; at each 

 side it is beginning to 

 grow in below, on the 

 surface of the yolk. 

 The space below the 

 hypoblast is the ali- 

 mentary cavity, the 

 ventral wall of which 

 is formed by the cells growing in at the sides. Between the 

 epiblast and hypoblast are placed scattered mesoblast cells, the 

 origin of which has not been clearly made out. 



In a later stage the openings of the two atrial tubes gradually 

 travel backwards, and at the same time approximate, till finally 



FIG. 12. 



A. SURFACE VIEW OF THE OVUM OF PYROSOMA 



NOT FAR ADVANCED IN DEVELOPMENT. The em- 

 bryonic structures are developed from a disc-like 

 blastoderm. 



B. TRANSVERSE SECTION THROUGH THE MIDDLE 



PART OF THE SAME BLASTODERM. 



at. atrial cavity ; hy. hypoblast ; n. nervous disc 

 in the region of the future Cyathozooid. 



