186 



/()()L()(iY 



SKOT. 



be connected with it only by a narrow n 

 The head and tail of the young Fish 



S:^^^^/^'^:;'t^A^>^%: 



^ 



^^x 



V . 



Fni. S44. — Tbreo views of the developing egg of an 

 Elasmobrancta., slunving the embrj-o, the blasto- 

 derm, and tlie vessels of the yolk-snc. Tlie shaded 

 part (l>\.) is the blastoderm, the white jiart the un- 

 covered yolk. A, young stage with the embryo still 

 attaehed at the edge of the blastoderm ; B, older 

 stage with the yolk not quite enclosed by the blasto- 

 derm ; C, stage after the complete closure of the yolk. 

 11. .arterial truijk.s of yolk-.sac ; hi. blastoderm ; r. 

 venous tr\inks of yolk-sac ; y. point of closure of the 

 yolk-blastopore ; .r, portion o* tlie blastodei-m out- 

 side the arterial sinus terminalis. (From Halfour.) 



Into these folds penetrate a series 



eck or yolk-stalk (Fig. 844). 



soon become differentiated, 

 and a series of involu- 

 tions at the sides of the 

 neck (Fig. 845) form the 

 branchial clefts and 

 spiracle. A number of very 

 delicate filaments (Figs. 

 845, 846) grow oat from 

 these apertures and be- 

 come greatly elongated ; 

 these are the pro- 

 visional gills, which 

 atrophy as the develop- 

 ment approaches com- 

 pletion, their bases alone 

 persisting to give rise 

 to the permanent gills. 

 The great development 

 of these gill-filaments in 

 the embryos of some vivi- 

 parous forms suggests 

 that, in addition to their 

 respiratory functions, they 

 may also serve as organs 

 for the absorption of 

 nutrient fluids secreted 

 by the villi of the uterine 

 wall.i The fins, both 

 paired and unpaired, ap- 

 pear as longitudinal 

 ridges of the ectoderm 

 enclosing mesoderm. In 

 some Elasmobranchs the 

 paired fins are at first 

 represented on each side 

 by a continuous ridge or 

 fold, which only subse- 

 quently becomes divided 

 into anterior and pos- 

 terior portions — the rudi- 

 ments respectively of the 

 pectoral and pelvic fins. 



of buds from the proto- 



' In a ppecie.s of Trygon a number of tlie villi of the nterus project into 

 the pharynx of the fa'tus throiigh the spiracles, and nourishment is pi'obably 

 received bj' this means. 



