PHYLUM CHORDATA 



171 



head, but, on the formation of the gill-clefts, a series of mesodermal 

 segments appear, the cells of which give rise to the muscles of the 

 branchial, hyoid, and mandibular arches, and probably also of the 

 palato-quadrate and the 



e y e - A 



By degrees the body of 

 the young fish becomes 

 moulded on the blasto- 

 derm. This is effected 

 by the formation of a 

 system of folds, anterior, 

 posterior, and lateral, 

 which grow inwards in 

 such a way as to separate 

 off the body of the em- 

 bryo from the rest of the 

 blastoderm enclosing the 

 yolk. As the folds ap- 

 proach one another in the 

 middle, underneath the 

 embryo, they come to 

 form a constriction con- 

 necting the body of the 

 embryo with the yolk 

 enclosed in the extra- 

 embryonic part of the 

 blastoderm. The process 

 may be imitated if we 

 pinch off a portion of a 

 ball of clay, leaving only 

 u narrow neck connecting 

 the pinched-ofP portion 

 with the rest. The body 

 of the embryo thus be- 

 comes folded off from the 

 yolk-sac and comes to be 

 connected with it only by 

 a narrow neck or yolk- 

 stalk (Fig. 796). 



The head and tail of 

 the young Fish soon be- 

 come differentiated, and 

 a series of involutions at 



FIG. 796. Three views of the developing egg of an 

 Elasmobranch, showing the embryo, the blasto- 

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

 part (bl.) 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.) 



the sides of the neck (Fig. 



797) form the branchial clefts and spiracle. A number of very 

 -delicate long filaments (Fig. 797) grow out from these apertures; 

 these are the provisional gills, which atrophy as the development 



