102 ON THE MORPHOLOGICAL CONSTITUTION OF 



pancreas, and lungs. The heart and pericardium are at first 

 comparatively large, project downwards, and only pass back- 

 wards at a comparatively late period into the interior of the 

 hsemal arches of the thoracic sclerotomes in reptiles, birds, 

 and mammals. The cephalic portion, or pharyngeal cul-de- 

 sac, on the other hand, does not present originally any traces 

 of the development of the hsemome. This may be to a certain 

 extent explained by the great comparative development of the 

 cephalic portion of what would have been formerly considered 

 the " serous layer" of the blastoderm a. The extremities of 

 the so-called "first visceral laminse" have in fact approached 

 one another below, before the apex of the cardiac tube has 

 advanced so far forwards as to communicate with them. The 

 precise conditions, however, which determine the formation of 

 the sclerous elements of the mandibular, hyoidean, and bran- 

 chial arches on the inside of the corresponding vascular arches, 

 remain to be ascertained by future inquiry. At present I can 

 only conceive of these conditions as in some way dependent 

 upon the developmental relations to which I have alluded. 



These relations of the haemal arches of the head must be 

 taken into consideration in determining the signification of the 

 branchial arches of the amphibian and fish. The division of 

 the sclerous system into dermo, neuro, and splanchno skeleton 

 was first systematically carried out by Cams. I was early 

 brought, by the study of the works of the philosophical and 

 ingenious Dresden anatomist, to adopt this threefold division 

 of the skeleton. I have latterly, however, been induced to 

 reject as untenable the doctrine of a splanchno-skeleton. I 

 believe it may be confidently asserted that no structure refer- 

 able in any way to the skeleton is developed in or around any 

 portion of the mucous layer of the vertebrate alimentary tube 

 beyond that part of it which belongs to the head ; in other 

 words, beyond the pharynx, or part perforated by the bran- 

 chial clefts. The mandibular, hyoidean, branchial, and pharyn- 



