GONADS. 



41 



In the region of the pharynx, the atrial cavity extends dorsal- 

 wards between the splanchnocoele and the ventral extension of 

 the myocoele (Fig. 29), and the splanchnocoele becomes cut up 

 by the gill-clefts into the sections of the coelom already described 

 as occurring in the primary pharyngeal bars (p. 24). The dorsal 

 and ventral regions of this part of the splanchnocoele furnish 

 the dorso-pharyngeal (sc) and endostylar coelom (ec) respect- 

 ively. 



The first myotome is developed from 

 the somites of the anterior pair, the so- 

 called collar somites (p. 34), which retain 

 their communication with the enteron 

 longer than the others. This communi- 

 cation on the left side becomes elongated 

 and gives rise to Hatschek's nephridium 

 {Fig. 9, 10). These somites send back ven- 

 tral extensions which lie in the developing 

 atrial folds. It has been suggested that 

 these give rise to the metapleural lymph 

 canals, but this has been denied. The 

 origin of the metapleural lymph canals 

 is not certainly known. 



Van Wijhe in his recent important 

 paper (op. cit.) states that the walls of 

 the collar cavity give rise to several 

 myotomes. It is not quite clear to us 

 whether this statement is based on em- 

 bryological study or not. He further 

 states that the second myotome of the 

 body is the anterior of these myotomes 

 which come from the collar-somite ; thus 

 implying that the walls of the head 

 cavities (preoral somite) give rise to a 

 myotome. So far as we know, the head 

 cavities do not give rise to myotomes. 

 The collar-coelom appears to give rise to 



EuS 



Fig. 28. — Transverse section through 

 a young Ampkioxus immediately 

 after metamorphosis, between tlie 

 atriopore and the anus, diagram- 

 matic (from Korschelt and Heider, 

 after Hatschek). 1 outer wall of 

 myotome (cutis layer) ; 2 muscles ; 

 3 fascia layer (outer wall of sclero- 

 tome ; 4 skeletogenous (inner) 

 wall of sclerotome ; 5 u 6 ventral 

 continuation of skeletogenous 

 layer and somatic wall of splanch- 

 nocoele ; 7 splanchnic ditto ; I 

 myocoele ; /' dorsal I" ventral fln- 

 space ; // splanchnocoele. 



the stomocoel and cavum epipterygium 

 of van Wijhe, who states that the cavum epipterygium communicates with 

 the metapleural canals. This confirms MacBride's statement that the 

 metapleural lymph canals are parts of the collar coelom. The cavum 

 epipterygium is also stated to communicate with the endostylar coelom 

 through the coelom in the first branchial arch, which, being part of the 

 splanchnocoele, it might reasonably be expected to do. The arrangement 

 of the parts of the coelom about the mouth as explained by van Wijhe is 

 complicated. 



The gonads are segmented in their origin. The generative 

 cells are first seen as the thickenings of the coelomic epithelium 

 at the ventral ends of the myotomes, on the anterior wall of the 



