228 



INVEETEBEATA 



CHAP. 



rucking up of the representative of the dorsal surface into an out- 

 standing fold, just as the growth of the endodermic rudiment in the 

 pathological blastulae of Echinus, produced by rearing the eggs in 

 warm water, leads to the formation of a gut rudiment which projects 

 from the surface of the blastula like the finger of a glove. Another 

 suggested explanation of reversion is that it is conditioned by the 

 pressure exerted on the egg by the tough chorion. The peculiarity 

 about the process is that whilst the dorsal surface increases in extent 

 it continues to form a part of the spherical surface of the egg ; nay 

 more, as it extends laterally it actually burrows under the mass of cells 

 which form the origin of the caudal lobe, and causes this structure to 

 appear as an outstanding projection. Kautsch points out, however, 

 that if reversion is to be attributed to the increase in length of the 

 dorsal surface of the embryo, the dorsal ectoderm cells should be under 



Fig. 176. — Transverse sections through two stages Fig. 177. — The embryo of Agdena 

 in the develojjment of the heart of Agelena when reversion is complete. (After 



labyrinthica. (After Kautsch.) Kishinouye. ) 



card, thickened -wall of apex of coelomic sac which coa;, coxal gland ; ster, stercoral pocket ; sp, 

 forms wall of heart (the reference line is a little too .spinnerets. Other letters as before, 



short) ; H, heart-cavity. 



lateral pressure and should assume a columnar shape, whereas they 

 are obviously passively stretched since they are exceedingly attenu- 

 ated. He concludes that no mere mechanical explanation of reversion 

 is possible. 



During the progress of reversion many changes take place. The 

 first abdominal segment, as well as the sixth, seventh, and eighth, 

 have by this time disappeared. The coeloipic sacs of the abdominal 

 region extend dorsally till they meet one another in the mid-dorsal 

 line. Below this point they do not meet, and so a space is enclosed 

 between their dorsal apices which is the cavity of the heart. In this 

 space are found many of the " fat " endoderm cells, alluded to above, 

 which thus form the first blood corpuscles. The walls of the heart 

 are formed by specially thickened regions of the walls of the coelomic 

 sacs {card, Fig. 176). Since the coelomic sacs do not fuse with their 

 successors, there occur gaps in the heart-wall between two segments. 



