REMARKS ON THE DEVELOPMENT OF AMPHIOXDS. 359 



for a few sections only. It is formed not as a groove at all, 

 but by the meeting together of the two free atrial walls, 

 which in younger individuals are well defined, and continued 

 for some distance backwards behind the atrium. Its mode 

 of formation is, therefore, strictly comparable to the account 

 given by me of the manner in which the main portion of the 

 atrium is formed. 

 To sum np — 



(1) The atrial cavity in Amphioxus is walled in by a pair 

 of ridges which may be termed atrial ridges, which appear 

 at a very early period of development as comparatively low, 

 broad elevations, each of which contains a cavity, which is 

 an extension of that of the first myotome (collar cavity) . 



(2) The ectoderm on the external side of these ridges be- 

 comes thickened, the cells composing the thickening become 

 clear and glassy, and eventually are hollowed out to form a 

 "lymph canal." My former statement as to the coelomic 

 nature of this lymph canal is therefore incorrect. The 

 thickenings containing the lymph canals are called meta- 

 pleurae. 



(3) The extensions of the collar cavities into tlie atrial 

 ridges become first separated off as the metapleural ccelom 

 on each side ; later this ccelomic space becomes converted into 

 a solid mass of cells, from which arise muscular fibres in the 

 neighbourhood of the gill openings, and almost certainly later 

 the subatrial muscle. This separation and solidification of 

 the metapleural coelom is coincident with an accentuation of 

 the atrial ridges. 



(4) The atrial ridges unite beneath the ventral surface of 

 the body, and enclose the atrium. 



(5) In the larva practically the whole sides and dorsal 

 portion of the pharynx represent merely the hyperpharyngeal 

 groove and the adjacent epithelium of the pharynx of the 

 adult, the whole of the branchial epithelium of the adult being 

 represented by a very narrow strip of the ventral wall of the 

 pharynx of the larva. The subsequent disproportionate 

 growth of this part of the pharynx of the larva and of the 



VOL. 43, PART 2. NEW SERIES. B B 



