20 



PYROSOMA. 



cells derived from the germinal disc, rapidly spreads over the sur- 

 face of the yolk, and becomes divided into two layers, the epiblast 

 and the hypoblast. At the same time it exhibits a distinction into a 

 central clearer and a peripheral more opaque region. At one end 

 of the blastoderm, which jfor 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 epiblastic 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 poste- 

 rior 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 sec- 

 tion, through about the 

 middle of this blastoderm, 

 is represented in fig. 1 2 B. 

 The epiblast is seen above. 

 On each side is the sec- 

 tion 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 ven- 

 tral wall of which is form- 

 ed by the cells growing 

 in at the sides. Between 

 the epiblast and hypoblast 

 are placed scattered meso- 

 blast 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 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. 



A. 



Fig. 12. 



Surface vikw of the ovum of Pyrosoma 

 NOT FAR ADVANCED IN DEVELOPMENT. The embry- 

 onic structures are developed from a disc-like blas- 

 toderm. 



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. 



