84 JOHN WARREN 



an embryo of 15 mm. (fig. 12), a slight hint of a paraphysis ap- 

 pears in the paraphysal arch. The postvelar arch has become 

 greatly elongated and thinner, while the epiphysal arch has 

 become much condensed with thicker walls. There is a long 

 pars intercalaris and the posterior commissure has continued 

 its encroachment therein. In figure 13, a picture of the median 

 section of the forebrain in an embryo of 23 mm., the arches 

 have passed out of their primitive condition and have assumed 

 their more advanced characteristics. The long extent of the 

 postvelar arch is noticeable and the pecuhar modification of its 

 velar end has commenced. The velum is small but distinct 

 and a small paraphysal outgrowth is seen in the much reduced 

 paraphysal arch. The epiphysis has been definitely formed and 

 the posterior commissure has developed forward through the 

 whole extent of the pars intercalaris up to the dorsal wall of the 

 epiphysis. No sign of a superior commissure could be seen in 

 the embryo, though Bailey (2) shows it in an embryo of 19 mm. 

 The pars intercalaris has now become much reduced in length 

 due to the increased development of the midbrain and corre- 

 sponds in this way to a similar process in lower vertebrates. 



2. Paraphysal arch and paraphysis 



Very few observations had been made on the pineal region in 

 human embryos until the appearance of Bailey's (2) paper in 

 1916. Selenka (19) gave the first description of the paraphysis 

 in opossum embryos, but showed no figures of the structure. 

 D'Erchia (5) has- described the paraphysis as a simple fold in 

 an embryo of about 30 mm., and Francotte (8) shows a picture 

 of a section through a tubular paraphysis in a three weeks 

 embryo. In a previous article, Warren (24) figure 39, there 

 appeared to be a distinct paraphysal outgrowth in a section of 

 an embryo of 28.8 mm., H. E. C. no. 1598, but as will appear 

 later this observation was incorrect. Bailey (2) described the 

 roof of the forebrain in three embryos of 19 mm., 28 mm., and 

 32 mm. respectively. His observations were based especially 

 on the development of the choroid plexuses. As regards a 



