^2 CHIM^.ROID FISHES AND THEIR DEVELOPMENT. 



constituted cells may divide, and by mitosis, although this is not of the usual type 

 {cf. ate/). In this connection, finally, in k a detail is given showing that megaspheres 

 as they pass into the yolk-entoderm present more or less evident mitosis, witness 

 the conditions vi and w {cf. also fig. 72 f at ni). Parenthetically, just below the 

 megaspheres here mentioned are vacuoles into which merocj'tes are passing. 



EARLY EMBRYOS FROM THE COMPLETE CLOSURE OF MEDULLARY FOLDS 

 TO OPENING OF GILL-CLEFTS. 



An earh' embryo attached to its blastoderm is shown in j^ate v, fig. 3S. This 

 may be contrasted with the stage of closing medullary folds shown in same plate, 

 fig. 37. In the blastoderm we observe that the spongy central area has increased 

 notably in size and that it has even extended to the anterior rim of the blastoderm. 

 We note also that as^-mmetry has made its appearance, the embryo now lying some- 

 what on its right side. The present blastoderm has increased more rapidly at its 

 •left, and here a lobe-like eminence is produced hindward over the j'olk. The 

 entire size of the blastoderm is scarcely larger than in the preceding figure. The 

 embryo is shown in detail, plate vi, as an opaque, fig. 41", and then as a transpar- 

 ent object, fig. 41''. In general this stage corresponds with Balfour's stage o in 

 shark; it differs, however, in the definiteness of its structures, for the anterior region 

 has already become quite highly differentiated in spite of the fact that the tail region 

 is still flattened out against the yolk and hardly protrudes beyond the rim of the 

 blastoderm. About 22 segments are present in this stage. The head rises above 

 the blastoderm and the divisions of the brain and the optic vesicles are formed, and 

 it is an evidence of the high specialization in development that the embryo of 

 this large-eyed form should possess large optic vesicles at this early period, /. c, 

 before the tail end of the body is established, — a fact of considerable interest 

 from the standpoint of embryonic adaptation. In this stage two gill-slits are 

 appearing, g^, ^". The region of the pronephros is marked out at/;;, the heart at 

 //, the anterior cardinal vessels at <:, and the vitello-intestinal at o. In the tail region 

 the neurenteric canal is distinctly seen at ;/. 



Details of the Present Embryo (Stage G). 



A series of selected transverse sections of this embrj^o may now be passed in 

 review to indicate the more prominent advances, figs. 73 a-uu. The anterior 

 sections a-d pass through the ectoderm inclosing the tip of the head and show a 

 conspicuous median infolding (recessus olfactorius impar) which in surface view 

 gives the appearance of separating a "forebrain" from a "right optic vesicle," the 

 sections having been cut in the plane indicated by the dotted line in plate vi, fig. \\^. 

 The next section (e) touches the distinct end of the central nervous system, the wall 

 of which is more extensively traversed in f and o. In 11, i, and j, the lumen of the 

 forebrain is traversed. In k and l, representing many sections, the cavities of 

 the optic vesicles appear, and we observe here closely apposed to the ventro- 

 median wall of the brain a mass of cells which in later sections is seen to constitute 

 the anterior end of both notochord and gut. In sections m and n this cell mass forms 

 a conspicuous ventral keel, in n the lumen of the gut first appearing. In o and ? 



