EMBHYOyiC DEVELOPMENT. 



129 



Strongly confirmed by the mode of development of the 

 praemandibular cavities in the Cyclostomes. 



In these fishes, von Kupf- 

 FER has shown that they 

 actually appear in the form 

 of a pair of diverticula from 

 the anterior extremity of 

 the archenteron (Fig. J2). 

 If a comparison be made 

 between Figs. 70 and 72, it 

 will be at once manifest how 

 close the correspondence is 

 between the mode of de- 

 velopment of the head-cavi- 

 ties in Amphioxus and in 

 Ammoccetes. In the Se- 

 lachians the similarity is 

 hardly less striking.^ 



E)tdostylc and Pigment 

 Granules. 



Fig. 72. — Horizontal projection of 

 pharynx and praeoral endodermic exten- 

 sion of a young Ammctirtes plaucri of 

 3^2 mm., reconstructed from a series of 

 transverse sections. (After Kui'FFER.) 



p.e. Pra?oraI endodermic extension 

 (prneorale Endodermtasche). pni. and ;;/. 

 Praemandibular and mandibular portions 

 of Iread-cavities. ph. Cavity of piiarynx. 

 /, 2, J. First three pairs of gill-pouches. 



N.B. — Kupffer considers it probable 

 that the mandibular as well as the pr:t- 

 mandibular cavities arise from the single 

 pair of endodermic diverticula. In the 

 course of the following pages I have 



referred chiefly to the praemandibular 

 In Fio" 64. there is to be cavities alone so as to avoid complica- 

 tions. 



noticed a vertically placed 



structure lying in front of and contiguous with the club- 

 shaped gland. It is a tract of very high cylindrical cells 

 forming part of the right wall of the alimentary canal in 



homologised with the praemandibular cavity (see below, V.). Kupffer regards 

 the praemandibular and mandibular head-cavities as rudimentary or meta- 

 morphosed gill-pouches. This deduction is entirely foreign to the standpoint 

 which I have adopted. The conclusion may seem plausible from the con- 

 ditions observed in Acipenser alone; but when these are regarded from a 

 comparative point of view, the deduction is. to my mind, unjustified. It should 

 be added that Kupft'er has shown that the head-cavities (praemandibular and 

 mandibular) of Acipenser also arise as endodermic pouches. 



