186 



ZOOLOGY 



SECT. 



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 their 

 respiratory functions, they 

 may also serve as organs 

 for the absorption of 

 nutrient fluids secreted 

 by the villi of the uterine 

 wall. 1 The fins, both 

 paired and unpaired, ap- 

 pear as longitudinal 

 ridges of the ectoderm 



Dping egg of an 

 ibryo, the blnsto- 



FIG. 844. Three views of the develo 

 Elasmobranch, showing the embry 

 derm, and the vessels of the yolk-sac. The shaded 

 part (W.) 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 ; bl. 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.) 



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 



rctoral and pelvic fins. 

 buds from the proto- 



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

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

 received by this means. 



