9 6 



CHIM/EROID FISHES AND THEIR DEVELOPMENT. 



we distinguish in this ventral cell mass a lower lumen-bearing area and above a 

 thickened mass, on either side of which, attached but not fused, lies a solid mass of 

 mesoblast. In Q we distinguish notochord and gut (, g); on either side of the 

 notochord the mesoblastic somite (s) bears a cavity. In R the mesoblastic sacs 

 are well separated from both notochord and gut, and the notochord itself, greatly 

 reduced in size, shows a compressed and almost longitudinally subdivided appear- 

 ance. In s, the body of the embryo is becoming flattened on its side; the lumen of 

 the gut is deep and narrow; closely apposed to its sides are the mesoblastic masses 

 whose lumen now becomes greatly reduced ; on the dorsal median wall of the gut 

 appear the beginnings of a subnotochordal rod. From this stage onward the lumen 

 of the central nervous system becomes notably reduced. In T the section passes 

 through the embryo in the plane where the neck region flattens out over the yolk. 

 Here we note the distinct subnotochordal rod (sn) and the flattening mesoblast 

 which now forms a delicate band almost surrounding the gut. In the surface view of 

 this region, on the other hand, only the thickened proximal ends of the mesoblast 

 masses can be distinguished. In u, where the neck is flattened out, the heart 

 appears at h ; and in the upper region of the gut we note the thickening of the wall 

 of the gill-slit, the cavity of which is seen in the preceding section at g 1 . In v, as 

 indeed in some of the earlier sections, a thickened neural crest appears at nc. In 

 w the body cavity (be) is becoming conspicuous. In x the somato- and splanchno- 

 pleure spread out widely peripherally; in the gut we notice in the thickening of the 

 lateral walls an out-bending for the second gill-slit (cf. in z, g"*} and in the cavity of 

 the gut in this and in many sections following we find masses of yolk. These 

 masses, sometimes small, as in sections z, AA, BB, EE, sometimes large, as in Y, cc, 

 DD, are unquestionably budded out (as in EE and HH) of the ventro-median wall of 

 the gut. On account of their abundance and range in size we can not conclude 

 that they are artifacts, but, on the other hand, if we regard them as normal 

 structures, it is natural to assume that they serve as food material, and are assimi- 

 lated by the gut in the usual way. This conclusion, simple as it seems, is none the 

 less difficult, since it attributes to Chimaera an embryological process which appears 

 to be unknown in the vertebrata and only remotely paralleled among invertebrates. 

 If, accordingly, we accept the present evidence, it follows that Chimsera is to be 

 regarded as the terminal member of an evolutional series, at one end of which were 

 forms whose yolk-laden cells contributed directly to the growth of the young; next 

 came those whose yolk-filled cells contributed indirectly to the growth of the young 

 through various processes, typically through the intervention of merocytes; and 

 finally, in Chimaera, the mode of nutrition by merocytes is supplemented by a still 

 more oblique process, i. e. , one which passes fragmented yolk material from the 

 zone of merocytes directly into the lumen of the gut. 



Continuing the sections: Inoc, and in many sections following, a wedge-shaped 

 mass of yolk material (w) is converging toward the ventro-median line of the gut 

 (v. also p. 76); in LL it becomes subdivided, and in MM appears a small recess which 

 may also contain this nutriment (? anlage of liver). In jj and in following 



