334 Mr. W. K. Parker on the Development of the [Feb. 13-, 



If the head had been straight, this drop of cartilage would have 

 reached its fore-end directly below the first nerves. 



I shall return to this part of the subject in the latter half of my 

 paper. 



The visceral or inferior arches of the head are as diverse from the 

 costal arches as the axial parts of the head are from those of the 

 body. 



That splitting of the mesoblast, which forms the " body-cavity," runs 

 high up into the muscle-plates ; but this upper part of the cavity 

 closes again, whilst, below, the ventral wall is permanently divided into 

 " splanchnopleure " and " somatopleure." 



I have corroborated Mr. Balfour's account of the extension of this 

 cavity into the head of the embryo of the Selachians by demonstrating 

 it in the head of the embryo lizard and turtle. 



To me this "head cavity" appears to be the equivalent of the 

 temporary upper part of the body cavity; this cephalic extension of 

 the cavity is also temporary. 



The cells lining these cavities in the head become transformed into 

 muscular fibres. 



Thus, there is a reversion of the ventral wall of the head into a 

 generalised condition before the visceral rods are developed. 



In the trunk the axial skeleton* is formed inside the upper diver- 

 ticulum of the body cavity, and therefore in the splanchnic layer; of 

 course the costal arches pass over the permanent body cavity into the 

 somatic layer. 



Mr. Balfour's view with regard to the visceral arches and branchial 

 pouches is, that they are all formed in a tract equivalent to the somatic 

 layer of the body. 



Yet the difference between the costal and visceral arches is very 

 great ; and the fact that there are two sets of them, external and 

 internal, separated by a large branchial space, does not lessen the diffi- 

 culty of harmonising these two sets of arches, the costal and the 

 visceral. 



We may, however, keep the term pleural for both, and divide this 

 "genus" into two "species" — visceral and costal — and the visceral 

 into two varieties, namely, external and internal. 



I suspect that there has been a secular differentiation of these 

 regions, and that the order in time has been — first, " extra-branchials " 

 round the huge branchial pharynx ; then the trabecule as a support to 

 the swelling neural axis; then the paired neural cartilages of the 

 spine ; after these, the intra-branchials ; then the costal arches ; and, 

 last of all, the limb-girdles and limbs. 



These deductions are not made at random, but by reflection upon 



* See Balfour, " Elasmobranchs," p. 133. 



