518 p. E. SMITH 



cavity will later appear, forming the anterior part of the adult 

 epiphysis. This indicates, I believe, that the epiphysis is double. 

 Both diverticula, morphologically, have distinct openings, but, 

 due to their proximity and to the drawing out of the epiphysial 

 arch, they finally come to have a common ostium leading into the 

 third ventricle. In this way what is apparently a single organ 

 is fundamentally double. That the posterior outgrowth arises 

 before the anterior one, strengthens this conclusion. As men- 

 tioned by most other workers the epiphysis touches directly the 

 body ectoderm, there being no intervening mesenchyme. The 

 habenular commissure is not yet evident. 



By Stage XI (figs. 3, 33, 39,) the paraphysis has formed an 

 extensive tube but, as yet, with no secondary diverticula. The 

 anterior paraphysial diverticulum has nearly disappeared, an 

 indication, only, of it being seen immediately cephalad to the 

 ostium of the paraphysis. The habenular commissure has become 

 very slightly evident, while caudad the posterior commissure is 

 prominent and is farther removed from the epiphysis than in the 

 earlier stages. This does not necessarily indicate, however, that 

 the growth of the commissure itself has been caudad. If this 

 were the case the commissure would belong to the roof of the mid- 

 brain. It indicates rather, I think, a lengthening of the region 

 inmiediately caudad to the epiphysis. In later stages (figs. 4,5) 

 the posterior commissure and the epiphysis again approach 

 each other because of the increase in size of both of these struc- 

 tures. The epiphysis has been placed in the first diencephalic 

 segment by several writers, and since the posterior commissure 

 later lies in contact with it, this relation has been taken as evidence 

 that the roof of the second diencephalic segment (synencephalon ) 

 does not lengthen. With this interpretation I can hardly agree. 

 The first appearance of the epiphysis is next to the di-mesence- 

 phalic constriction. Then the portion of the roof between it and 

 the posterior commissure lengthens as does the rest of the roof 

 of the diencephalon, thus causing the separation of these two 

 structures. It would be peculiar if a small part of the dien- 

 cephalon, such as is represented by the synencephalon of Kupffer, 

 remained alone free from growth while all other parts of the brain 

 were undergoing such rapid extension. 



