520 



ORR. [Vol. I. 



the lateral parts of the head has enclosed, the hypophysis lat- 

 erally, while its median roof has remained in its primitive 

 position in contact with the same part of the brain where it 

 first appeared. In the embryo of series C the hypophysis appears 

 as a lateral, slit-like depression in the roof of the mouth. Later 

 stages show that the growth of the surrounding parts gradually 

 constricts the opening of the hypophysis to the mouth. The 

 hypophysis appears then as a laterally distended body with 

 extremities turned slightly upward, according with the curved 

 surface of the brain, against which it lies. 



The head-cavities having first appeared as already described, 

 become large oval cavities, with walls of a distinct, compact, 

 epithelial nature. Their position and relations may be seen at 

 //r, in Figs. 2,C, 34, C, 35,C,36, C, and 48, D. The nerves of the 

 third pair enter their dorsal posterior walls {CZ] Fig. 34, C). 

 At a late period in the duration of the head-cavities, at that end 

 of each cavity nearest the mouth, a transverse constriction 

 appears in the median wall, as if the cavity were to be divided 

 into two, — one smaller anterior and one larger posterior cavity. 

 Whether such a division really takes place I have been unable 

 to prove, for very soon after the appearance of the constriction 

 the whole cavity seems to disappear, its walls being converted 

 into muscle. The constriction is interesting, as corresponding 

 with a similar condition of the first head-cavities in the elasmo- 

 branch fishes.^ 



The coelenteric zone continues to exist for a considerable 

 time, uniting the anterior ends of the head-cavities; it is still 

 visible in stage C {n Illy Fig. 34, C). Figs. 43, C, and 44, C, 

 PI. XV., represent two successive sections in the region of the 

 coelenteric zone of the embryo figured at 2, C. In Fig. 43, C, 

 a solid band of cells {CZ) connects the head-cavities {HC) 

 above the hypophysis {Hp/i) ; and in the posterior section 

 (44, C) is seen the anterior tip of the notochord (A^) joining in 

 the middle with the band {CZ). I have not discovered any 

 later trace of the coelenteric zone. Im.mediately after this period 

 it seems to become absorbed. 



The preceding statements describe what seems to me to be 

 the most common manner of development of the anterior end 



' Balfour. — A Monograph on the Development of Elasmobranch Fishes. London, 

 iS-8. 



