634 



INVEKTEBEATA 



CHAP. 



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- 

 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 



&' 



arises 



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 ectodernial 

 grooves are formed which are 

 the atrial imaginations. 

 They become largely separated 

 from the exterior so as to form 

 tubes with narrow openings 

 directed dorsally. As growth 

 per S oes 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 



FIG. 461. Two embryos of P i /rosomagi ! /<(ii/<'iim, commun i ca tes With the 

 viewed from the dorsal surface in order . . -, 



to show the formation of organs in the pharynx, On its lUlier Side, 



Cyathozooid. (After Julin. ) 



A, younger embryo. B, older embryo \vliii-li lias 

 already formed a stolon, at, common atrium formed by 

 bhe junction of the two atrial tubes; nt.I, left atrial 

 tube; nt.n, obliterated portion of the left atrial tube; 



n 



by a narrow slit which is a 



rudimentary gill-Slit. 



Tl-.,-> nn-winavrliiim >ii-iut' fl 

 riCardlUm .1. Lbtb a, 



tWO Ventral OlltrOVVtllS OI tlie 



nt.r, right atrial tube; en<7*/, endostyle ; H, heart ; n.p, pharynx which, 



' 



nerve-plate; per, pericardial sac ; S toJ, stolon. 



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 



