88 CHiM^ROiD Fishes and their development. 



In summary: The evidence which is thus provided strent;thens the conckision 

 that in the gastrulation of Chimsera amitosis is not to be interpreted in accordance 

 with the current view, i.e., as a process of decadent cell division. It is conditioned, 

 rather, by rapid growth and multiplication of nuclei, since its products may resume 

 mitosis when the usual rate of cellular division is attained. Moreover, the products of 

 amitotic division in the blastoderm of Chimsera, are too many and too widely scat- 

 tered to warrant the belief that their cellular descendants can plaj^ no part in 

 producing permanent organs 



Later Gastrula. Embryo With Partly Closed Medullary Folds. 



This stage is figured in surface view, plate v, fig. 37, and enlarged, viewed as 

 a transparent object, in plate vi, fig. 41. It corresponds approximateh' with Bal- 

 four's stage F in the shark. 



Comparing the blastoderm of this with the preceding stage, we find that it has 

 increased but little in size. The spongy region, however, which occupies its central 

 portion appears more prominently, and we observe a noteworth}' thickening in the 

 region of mesoblast (gastral) extending outward on either side of the embryo. The 

 details of the embryo are well seen in a toto preparation. The medullary folds arch 

 over and meet in the median line, fusing in the posterior third of the embryo's length. 

 In front of this, after a slight interruption, the folds meet again, then diverge to a 

 degree suggesting the corresponding stage of shark. The tail folds are conspicuous 

 at this stage, and we observe that the gut has arched upward, a transverse line 

 showing where a neurenteric canal is to open below. On either side in this region 

 the mesoblast is thickened, fading away laterally. Here are forming the extensive 

 caudal veins. Other vascular details are shown in the antero-median vessel 

 (apparenth' vitelline vein) which appears immediatel}' in front of the head and 

 spreads out widely over the blood-pi"oducing region. We note also transverse larger 

 vessels, the vitello-intestinal, extending outward on either side to about an equal 

 distance. Gastral mesoblast is conspicuous in this stage; in this may Ije traced 

 about a dozen somites, the anterior ones extending far forward. 



Details of Foregoing Stage, Corresponding to Balfour's Stage F. 



Sections are shown in fig. 72 a-k ]iassing through the blastoderm in a plane 

 transverse to the axis of the embryo. In the first, which passes through the tail 

 region of the embryo, we observe that the mesoblast bands (iiies) are continuous 

 with the entoderm not in the region adjacent to the notochord Init marginallj' {cf. 

 the view of Graham Kerr as to this place of origin in the vertebrate gut pouches); 

 near by the entoderm (cni) thickens conspicuously, then thins again as it passes 

 into the notochord. Only at the open notch between the tail folds does the lumen 

 of the nerve tube pass over into the wide space (cf. fig. 71 a) which is coming to 

 form the cavity of the gut. It will be seen that it is especially the thickening of 

 the entoderm and the constricted origin of the mesoderm which in the transparent 

 preparation (plate vi, fig. 41) causes the appearance of a dark band in the region of 

 the tail folds of the embryo. In fig. 72 b similar conditions in gastral mesoblast and 



