634 



INVEETEBEATA 



CHAP. 



^atr 



In later stages of development the archenteric cavity acquires a 

 floor of cells, produced mainly by the increase in number and adhesion 



of the yolk cells, which repre- 

 (j\ ..^r"^® ©"'>-. sent the white cells at the 



^^ ^^ ^ lower pole of the i frog's egg. 



Meantime the ectoderm, 

 extending out from the 

 anterior edge, i.e. the opposite 

 side of the blastoderm to that 

 of the region x, grows as a 

 simple sheet of cells over the 

 yolk, and so eventually the 

 blastopore is closed. The arch- 

 enteron becomes the pharynx, 

 no tail endoderm being formed. 

 The formation of organs 

 is now begun. The nervous 

 system arises as a median 

 dorsal invagination of ecto- 

 derm in front of the region x. 

 Behind it a median pit arises, 

 which is the rudiment of the 

 cloaca or median atrial open- 

 ing. On the sides of the 

 pharynx two ectodermal 

 grooves are formed which are 

 the atrial invaginations. 

 They become largely separated 

 from the exterior so as to form 

 tubes with narrow openings 

 directed dorsally. As growth 

 goes on their openings are 

 forced farther and farther 

 upwards, and become involved 

 in the median invagination, 

 alluded to above, which forms 

 the cloaca. Each atrial tube 

 communicates with the 

 pharynx, on its inner 'side, 

 by a narrow slit which is a 

 rudimentary gill-slit. 



The pericardium arises as 

 two ventral outgrowths of the 

 pharynx, which, however, 

 speedily fuse into one and 

 constitute a single vesicle. The single vesicle sends out two pro- 

 longations, a right and a left ; of these the left becomes solid and 

 atrophies, but the right swells and forms the persisting heart 



per 



Fig. 461. — Two embryos of Pyrosoma giganteum, 



viewed from the dorsal surface in order 



to show the formation of organs in the 



Cyathozooid. (After Julin. ) 



A, younger embryo. B, older embryo which has 



already formed a stolon, at, common atrium formed by 



the junction of the two atrial tubes ; at. I, left atrial 



tube ; at.U, obliterated portion of the left atrial tube ; 



at.T, right atrial tube ; endst, endostyle ; H, heart ; n.ji, 



nerve-plate ; jier, pericardial sac ; stol, stolon. 



