Reproduction 



275 



Ectoderm Notochord 



Neurenteric 

 canal 



Proctodeum 



Primitive 

 renal duct 



Genital ridge' 



Coelom 



Alimentary 

 tube 



Hypophysis 

 Stomodeum 



Hepatic Heart 

 diverticulum 



Gill cleft 

 Pericardial 

 cavity 



Fig. 228. Stereograms illustrating early stages in the development of a chordate. 

 The figures are generalized, but in some particulars are based on embryonic stages 

 in such "lower" vertebrates as a shark or amphibian. (A) Shows an earlier and (B) 

 a later stage in the differentiation of the mesoderm. In (C) an embryo at about the 

 stage of (B) is represented as cut, except in a narrow midtrunk zone, in sagittal 

 section. 



furrow of the endoderm bulges outward and meets a similar but shal- 

 lower pouch or furrow which the ectoderm pushes inward. The result- 

 ing two-layer membrane is then obliterated by the same process which 

 removes the oral membrane, leaving a free passage between the pharynx 

 cavity and the exterior. Vascular complications of the endodermal 

 lining of these clefts produce internal gills — although it is possible 

 that some so-called "internal gills" are derived from ingrowing ecto- 

 derm. External gills are ectodermal structures developed in close rela- 

 tion to the external apertures of pharyngeal clefts. The number of 

 pharyngeal pouches ranges from 14 pairs in some round-mouthed eels 

 (Myxine) to four or five pairs of more or less incompletely developed 

 pouches in reptiles, birds, and mammals. Those of the first pair become 

 the spiracles of sharklike fishes and appear in a modified way in the 



