EMBR 1 'OXIC DE VEL OPMENT. 



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. 72). 

 If a comparison be made 

 between Figs. 70 and 72, it 

 will be at once manifest how 

 close the correspondence is 



Fig. 72. Horizontal projection of 



between the mode Of de- pharynx and prasoral endodermic exten- 

 c , , . sion of a young Ammocastes planeri of 



VClopment of the head-cavi- ^ mm _ ^constructed from a series of 

 ties in AmphioxUS and in transverse sections. (After KUPFFER.) 



p.e. Prasoral endodermic extension 



AmmOCOeteS. In the Se- (prseoraleEndodermtasche). pm. and m. 

 , ., . Prcemandibular and mandibular portions 



ltV 1S of head-cavities, ph. Cavity of pharynx. 

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



N.B. Kupffer considers it probable 

 that the mandibular as well as the prae- 

 mandibular cavities arise from the single 

 pair of endodermic diverticula. In the 

 course of the following pages I have 

 referred chiefly to the prasmandibular 

 In Fi~ 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 prcemandibular cavity (see below, V.) . Kupffer regards 

 the pnemandibular 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 Kupffer has shown that the head-cavities (prsemandibular and 

 mandibular) of Acipenser also arise as endodermic pouches. 



hardly less striking. 5 



Endo style and Pigment 

 Granules. 



