1901.] NATURAL SCIENCES OF PHILADELPHIA. 505 



yolk-mass to the body are not shown, since the head hides a part 

 of the yolk-raass, and since the head is cut obliquely and so 

 appears larger than it would he in strictly median section. 



But the relations of the intestine to the yolk-mass are more 

 clearly shown in the camera drawings 4 and 5, which are dorsal 

 portions of cross-sections of the larva shown in its entirety in 

 fig. 1. Fig. 4 is in a plane behind the gill region, and shows how 

 the intestine (Int.) is connected with the yolk-sphere {Yk.'). 

 The epitheMum of the intestine, the nuclei of which are shown as 

 black spots, expands ventrally into ihe yolk-sphere on the dorsal 

 aspect of the latter. The epithelium of the intestine ends abruptly 

 against the peripheral layer of the yolk-sphere, and does not pass 

 over into it gradually. The nuclei of the yolk-sphere are much 

 smaller and peripherally placed, and in places are found small 

 clusters of nucleated cells. The yolk-mass of this larva shows 

 that the cell boundaries of the yolk-cells have disappeared, and 

 consequently the latter are undergoing regressive changes ; the 

 yolk appears as a mass of globules of different volumes suspended 

 in a structureless fluid and without nuclei. Fig. 5 represents the 

 dorsal half of a cross-section through the same larva, in a plane 

 about half-way between the anterior and posterior limbs. In 

 this region there is no tubular intestine, nor any open groove of 

 intestinal epithelium upon the yolk-sphere, the entoderm being 

 represented simply by the yolk-sphere (Yk.), the small nuclei of 

 which are on its periphery. 



These sections make clear the nature of the yolk-mass, and its 

 relation to the intestine. From the mouth to the commencement of 

 the small intestine the alimentary tract is tubular, and has the same 

 appearance in* its epithelial lining as in like stages of other Ba- 

 trachia; the same is true also for the rectal region of the alimen- 

 tary tract, which is likewise tubular. But the middle region of 

 the intestine is composed of the yolk-mass, which in the earlier 

 stage shown in fig. 3 is made up of large yolk-cells, and in the 

 latter stage of figs. 4 and 5 of a mass of yolk-globules with a 

 peripheral layer of small nucleated cells. Accordingly the large 

 yolk-sphere is not a yolk-sac, since it is an integral part of the 

 intestine. 



The larva shown in fig. 1, of which figs. 4 and 5 represent sec- 

 tions, is in quite an advanced stage. Externally can be seen the 



