50 GENERAL GROWTH OF THE EMBRYO. 



Owing to the opacity of the embryo, the muscle-plates are only 

 indistinctly indicated in fig. 28 F, and no other features of the meso- 

 blast are to be seen. 



The mouth is now a deep pit, the hind borders of which are 

 almost completely formed by a thickening in front of the first 

 branchial or visceral cleft, which may be called the first branchial arch 

 or mandibular arch. 



Four branchial clefts are now visible, all of which are open to the 

 exterior, but in the embryo, viewed as a transparent 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 thicken- 

 ing 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 ex- 

 ternal gills. The peculiar ventral curvature of the anterior end of the 

 notochord {ch) 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 cesophagus 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 cesophagus is present. It 

 appears not impossible that this feature in the cesophagus may be connected 

 with the fact that in the ancestors of the present types the cesophagus 

 was perforated by gill slits ; and that in the process of embryonic abbrevia- 

 tion the stage with the perforated cesophagus became replaced by a stage 

 with a cord of indifferent cells (the cesopliagiis being in the embryo quite 

 functionless) out of which the non-perforated cesophagus 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 characteristic 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 represented 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 diminution 

 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. 



The first cleft is no longer similar to the rest, but by the closure 

 of the lower part has commenced to be metamorphosed into the 

 spiracle. 



