ELASMOBRANCHII. 



61 



object, two more, not open to the exterior, are visible behind the 

 last of these. 



Between each of these and behind the last one there is 

 a thickening of the mesoblast which gives rise to a branchial 

 arch. The arch between the first and second cleft is known as 

 the hyoid arch. 



Fig. 29 B is a representation of the head of a slightly older 

 embryo in which papillae may be seen in the front wall of the 

 second, third, and fourth branchial clefts : these papillae are the 

 commencements of filiform processes which grow out from the 

 gill-clefts and form external gills. The peculiar ventral curva- 

 ture of the anterior end of the notochord (cJi) both in this and in 

 the preceding figure deserves notice. 



A peculiar feature in the anatomy makes its appearance at this period, 

 viz. the replacement of the original hollow oesophagus by a solid cord of 

 cells (fig. 23 A, ces) in which a lumen does not reappear till very much later. 

 I have found that in some Teleostei (the Salmon) long after they are 

 hatched a similar solidity in the oesophagus is present. It appears not 

 impossible that this feature in the oesophagus may be connected with the 

 fact that in the ancestors of the present types the oesophagus was perforated 

 by gill slits ; and that in the process of embryonic abbreviation the stage 

 with the perforated oesophagus became replaced by a stage with a cord of 

 indifferent cells (the oesophagus being in the embryo quite functionless) out 

 of which the non-perforated oesophagus was directly formed. In the higher 

 types the process of development appears to have become quite direct. 



By this stage all the parts of the embryo have become 

 established, and in the succeeding stages the features character- 

 istic of the genus and species are gradually acquired. 



Two embryos of Scyllium are represented in fig. 28 G 

 and H, the head and anterior part of the trunk being repre- 

 sented in fig. G, and the whole embryo at a much later stage in 

 fig. H. 



In both of these, and especially in the second, an apparent 

 diminution of the cranial flexure is very marked. This diminu- 

 tion is due to the increase in the size of the cerebral hemispheres, 

 which grow upwards and forwards, and press the original fore- 

 brain against the mid-brain behind. 



In fig. G the rudiments of the nasal sacks are clearly visible 

 as small open pits. 



