274 BASHFORD DEAN. 



Somites are prominent^ ps. Pronephric tubules are apparent 

 in connection with all the mesoblastic somites, pn, each nar- 

 rowing out laterally into a delicate contorted tube, but as yet 

 there has appeared no pronephric duct. In the gill region 

 about the normal number of gill-slits have now been formed. 

 The number increases from behind, but remarkably enough 

 those latest formed (i. e. hindmost) are largest. The gill 

 region is marked out clearly on either side of the embryo as a 

 conspicuous lappet-shaped outgrowth. 



Late Embryo (figs. 6, 7). — Continued growth has carried 

 the head of the embryo around the animal pole as far on the 

 ventral side as past the rim of the operculum ; the tail, now 

 narrow and tapering, has elongated until it has almost reached 

 the hind end of the egg. Head, neck, and tail are separated 

 below from the yolk-sac, although they lie in grooves, and are 

 deeply sunken into it. Again, a marginal artery, da, separates 

 the trunk from the yolk-sac. Somites are now clearly marked ; 

 pronephric structures are prominent, pn, and a segmental 

 duct, SD, is found. In the head region the principal advances 

 include the enlarged size of the nasal pouch, n, the appearance 

 of supporting tissue and of mouth, the latter originating as a 

 single invagination. This is very definitely shown in one of 

 the, writer's preparations, who thus difiers from the conclusion 

 of Price as to its probable origin as a paired structure. The 

 gill-slits at this stage, gs, may be seen through the back 

 when this region is viewed as a transparent object. Since the 

 last stage these have been drawn forward, and at the same 

 time inward toward the ventro-median line by the growth of 

 the head. The heart is at this stage a well-marked tube, 

 passing straight toward the head from the hinder pole of the 

 egg ; on either side it receives asymmetrically the vessels 

 from the yolk region, carrying the venous blood to the gills. 

 The vessels are very conspicuous in the living object, the 

 crimson threads in brilliant contrast to the rich yellow of the 

 underlying yolk. 



Latest Embryo (fig. 8). — The figure illustrates the latest 

 stage in the writer^s material. The young Bdellostoma 



