180 



ZOOLOGY 



SECT. 



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

 dermal segments appear, the cells of which give rise to the cartilages 

 and muscles of the branchial, hyoid, and mandibular arches, and 

 probably also of the palatoquadrate and the eye. 



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 in- 

 wards in such a way as to 

 separate off the body of 

 the embryo 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 connecting 

 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 a 

 narrow neck connecting 

 the pinched-off portion 

 with the rest. The body 

 of the embryo is thus 

 gradually folded off from 

 the yolk-sac and comes to be 

 connected with it only by 

 a narrow neck or yolk- 

 stalk (Fig. 855). The head 



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



part (bl.) is the blastoderm, the white part the un- i-mrWan rliffprpntifl- 



covered yolk. A, young stage with the embryo still SOOn Undergo < 



attached at the edge of the blastoderm; B, older + n anc l a series of involu- 



stage with the yolk not quite enclosed by the blasto- * " A "* 



derm ; C, stage after the complete closure of the yolk, tions at the SldCS OI the neCK 



a. arterial trunks of yolk-sac; bl. blastoderm; v. _,. QKf >\ f * ^ -J-V, 



venous trunks of yolk-sac ; y. point of closure of the ^r Ig. oOO ) IOIm UJ 



yolk-blastopore ; x, portion of the blastoderm out- -i -i r l p ffQ ~r\(\ 



side the arterial sinus terminalis. (From Balfour.) CHiai I 



A number of very delicate 



filaments (Figs. 856, 857) grow out from these apertures and 

 become greatly elongated ; these are the provisional gills, which 

 atrophy as the development approaches completion, their bases 

 alone persisting to give rise to the permanent gills. The great 



FIG. 855. Three views of the developing egg of an 

 Elasmobranch, showing the embryo,jbhe blasto- 



