DEVELOPMKIS'T OF TflE OALll'ORNIAN IIAG-FJSH. 273 



the fused lips of the blastopore^ whose fissure is still to be 

 made out behind the tail region, bp. Anteriorly the long 

 narrow brain mass, b, is indicated, scarcely deeper and wider 

 than the myelon into which it merges. No canalis centralis 

 can be determined in surface view. Notochord and meso- 

 blastic somites have not yet arisen. A mesoblastic thickening 

 is apparent in surface view only at *. 



Early Embryo (fig. 4). — This embryo is scarcely longer 

 than the earlier one, but it is notably broader, and it has made 

 marked advances in organogeny. Anteriorly the neural cord 

 has acquired a lumen ; its walls, thickened and folded asym- 

 metrically, mark out vaguely the divisions of the brain. Au- 

 ditory vesicles, au, are prominent ; optic vesicles present but 

 very indistinct. Mesoblastic somites, PS, about ninety in 

 number, appear close to the neural cord, and extend sideways 

 but a very short distance. They are here differentiated in 

 situ, but surface view cannot demonstrate definitely that gut 

 cavities are present. 



Interesting is the condition of the fore-gut, which now 

 dilates under the definite head region, and is already, as far as 

 one can judge from surface view, pierced with several gill-slits. 

 The heart, h, is vaguely indicated in front of the head, extend- 

 ing directly forward. 



Moderately Early Embryo (fig. 5). — The trunk has 

 become outlined in this stage ; on each side it is separated 

 from the yolk region by a marginal artery which passes back- 

 ward as it breaks into branches in the neighbourhood of the 

 tail. Body length has somewhat increased, due apparently to 

 growth in both directions, the head now extending under the 

 operculum, the tail somewhat nearer the opposite pole of the 

 egg. Advances in the development of the neural cord include 

 anteriorly the enlargement and definite outlining of the brain 

 parts which have now grown under the auditory vesicles and 

 over the eyes, and the now prominent and fused nasal pouches, 

 and posteriorly the extension of the canalis centralis into the 

 region of the tail. The asymmetry of the foldings of the 

 neural tube is now visible only in the region of the medulla. 



