22 ROYAL SOCIETY OF CANADA 



Attention must be called to the contrast in the method of inter- 

 ference in the head-region of my em])ryo and that in Kaestner's (No. 6, 

 Taf. VII) where the ventral surfaces have interfered more than the 

 dorsal, the result being a single heart and a double brain, instead of 

 a double heart and a single brain (cf. my figure 9). The plane of 

 interference becomes caudad more and more truly sagittal, so that the 

 chordae, at first widely divergent (Fig. 10), eventually fuse, (Fig. 13). 



I now proceed to the description of the various systems of organs. 



Nervous System. 



As a starting-point, I select section 12 (Fig. 5) through the region 

 of the optic vesicles. It is easy to understand how the condition here 

 pictured is arrived at if we proceed from the normal state as seen in 

 Duval's Figs. 353 and 251. The two embryos have been inclined witli 

 their dorsal surfaces towards each other, and have interfered in such 

 a way that the right and left lips of the neural groove of the one, have 

 fused with the right and left lips of that of the other. In this way, 

 no room is left for the complete development of the "' median " optic 

 vesicles which, consequently, are very minute (ov'). The points of 

 fusion are still noticeable and it is obvions that that of the left and 

 right lips of the right and left components respectively (which now form 

 the floor of the composite neural canal), is less complete, in such a way 

 that some mesoderm cells have intruded into the neural canal at this 

 point. The double character of the neural canal is brought strongly 

 out by the two infundibula v/hich diverge laterally towards the two 

 blmd foregut ends (ph.) beneath which the slightly ■ thickened patche? 

 of ectoderm already indicate the hypophyses. 



It is less easy to interpret the preceding sections (Figs. 1 to 4), 

 but if two components such as are represented in Duval's Fig. 252 

 have interfered in such a way as materially to reduce in size the con- 

 tiguous halves, then it becomes apparent that the convex floor of the 

 composite neural canal in figure 4 is formed of the left and right 

 brain-halves of the right and left components which have fused in 

 the region of their dorsal neural sutures, while their ventral sutures 

 are still widely separated. Still further forward (Fig. 3) these brain- 

 halves are fused so that the most anterior end of the neural canal 

 (Figs 1 and 2) is formed of the lateral brain-halves only of the two 

 components. It is noticeable that the separation of the brain from the 

 ectoderm has apparently taken place sooner than is normal (No. 3, 

 Vol. 2, page 252). 



In the diencephalic region (Fig. 6) the brain is much compressed 

 from side to side, but it soon widens out into the mid-brain (Fig. 7). 



