186 



ZOOLOGY 



be connected with it only by a narrow neck or yolk-stalk (Fig. 844). 



The head and tail of the young Fish 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 tlieir 

 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 subfe- 

 quently becomes divided 

 into anterior and po?- 

 terior portions — the rudi- 

 ments respectively of the 

 pectoral and pelvic fins, 

 of buds from the proto- 



^ In a species of Trygon a number of the villi of the uterus project into 

 the pharynx of the foetus through the spiracles, and nourishment is probably 

 received by this means. 



•t 



^K;i+t^,v 



-^ '"s 



Fio. 844. — Three views of the developing egg of an 

 Elasmobrancli; showing the embrj-o, the blasto- 

 derm, and the vessels of the yolk-snc. The shnded 

 part (hi.) is the blastoderm, the white part the un- 

 covered yolk. A, young stage with the embryo still 

 attached 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. 

 a. arterial trunks of yolk-sac ; hi. blastoderm ; v. 

 venous trunks of yolk-sac ; y. point of closure of the 

 yolk-blastopore ; x, portion of the blastoderm out- 

 side the arterial sinus terminalis. (From Balfour.) 



Into these folds penetrate a series 



