166 ALICE JOHNSON. 



begins to bud out behind it, at a time when about ten meso- 

 blastic somites have been formed, as a small conical knob whose 

 blunt apex points forwards, and the tail has become very dis- 

 tinct in the stage represented in fig. 13, when there are about 

 eighteen somites, and the rudiments of the sense organs, cere- 

 bral vesicles, visceral arches, &c., have appeared. 



The blastopore leads into the hind gut, whose cavity is here 

 broad, but very shallow (vide fig. 8). At a greater distance 

 from the blastopore the cavity becomes much narrower and no 

 deeper (vide fig. 6), so that it is very difficult to follow it in. 

 transverse sections. In the longitudinal sections, however, its 

 continuity is quite apparent (vide figs. 11, 12, 13). Fig. 14 

 represents a transverse section, showing the open blastopore at 

 a time before the tail is formed, and figs. 15, 16, 17, a series of 

 transverse sections, showing the passage of the blastopore into 

 the hind gut at a considerably later stage with a very distinct 

 tail. 



I find no stage at which the blastopore is closed. 



B. Historical. — Scott and Osborn^ describe a posterior 

 dilatation of the medullary canal, the sinus rhomboidalis, 

 which remains open for some time after the rest of the canal is 

 closed. They say that its folds enclose the blastopore, and, 

 therefore, when they come together, a neurenteric canal is 

 formed. Their account of the exact date of the closure of the 

 sinus rhomboidalis is a little obscure, but seems to indicate 

 that it takes place while the number of mesoblastic somites is 

 quite small, and before the rudiments of the visceral arches 

 and of the tail have appeared. 



Hertwig2 figures an open blastopore at a slightly later stage 

 than this, but he describes it as being situated at the end of a 

 small conical process, which, judging from his surface views of 

 the embryo, one would take to be the tail. 



1 W. B. Scott and H. F. Osborn, " On the Early Development of the 

 Common Newt," this Journal, October, 1879. 



2 0. Hertwig, ' Die Entwicklung des Mittleren Keimblattes der Wirbel- 

 thiere,' Jena, 1881. 



