So-called Mesectoderm in Pctromyzon. 



461 



A B 



t-fig. 1. — A and B show frontal sections, of which B shows a little further advanced stage. C represents a 

 transverse section through a postotic visceral arch of a much advanced larva. The nuclei of the mesecto- 

 derm cells and the structures derived from the mesectoderm are shaded, a.m., muscular artery ; ao., aorta ; 

 be., branchial cleft ; bcl., branchioccele ; eh., chorda ; ct, connective tissue ; ec.,. ectoderm ; eg., epibranchial 

 ganglion ; en., entoderm ; ep., epidermis ; km., hypoglossal muscle ; ma., mesodermal visceral arch ; m.ad., 

 adductor muscle ; me., medullary canal ; me., mesectoderm ; ph., pharynx ; r.bc., rudiment of branchial 

 cleft ; r.va., rudiment of vascular arch ; r.vca., rudiment of visceral cartilaginous arch ; s.ct., subchordal 

 connective tissue ; sm., scleromyotome ; ta., truncus arteriosus ; thy., thyroid gland ; va., visceral vascular 

 arch ; v.c., cardinal vein ; vcb., visceral cartilaginous bar ; v.ji., vena jugularis impar ; vp., visceral pouch. 



in my paper above referred to (145) the ventral series of cephalic ganglia. 

 The epibranchial placodes are not only cut off from their mother-layer, but 

 have been already transformed into the definitive nervous system of the 

 branchial apparatus, when the ectoderm commences to be thickened for 

 the gill-clefts. 



About the fate of the foremost placode, which is situated, as seen in 

 text-fig. 14 by Schalk, immediately behind the optic cup, the author gives 

 no definite account. Judging from the position in which it is found, and 

 from what he says about it, the placode must be taken to be the rudiment 

 of the lens which belongs to the trigeminal region. Here the circumstances 

 are not so simple as shown in the figure. The ophthalmic and trigeminal 

 placodes and the placode for the lens have coalesced at their bases and are 

 distinguishable from one another only by the divergence of their distal 

 parts, and they embrace between them the second mesodermic somite and 

 a part of its mandibular fold, being closely apposed to one another. The 



