Bawden, Neurological Laboratory Notes. 47 



near the middle of the roof of the diencephalon. The pharyn- 

 geal sac in this stage is situated immediately behind the hypoph- 

 ysis, the interval between the two being filled for the most part 

 with the same dense variety of connective tissue which occupies 

 the site of the future basis cranii. [See also fig. 2.] There is, 

 however, a small opening or blood sinus which extends nearly 

 parallel to the axis of the hypophysis for the whole length of the 

 latter. 



The pharyngeal velum is absent at this stage; but must 

 have been situated immediately caudad of the hypophysis, so 

 that the sac in question belongs to the hypoblastic tissue of the 

 alimentary tract. The sac is simple and in this respect differs 

 from the same organ in the opossum. Its cavity, which at first 

 is narrow, expands dorsally and the walls rapidly thicken, giving 

 a bulb-like configuration to the whole organ. There is a slight 

 tendency to lobate form, but no actual diverticles at this stage. 

 [Fig. 2.] The epithelium is single-layered, vacuolated toward 

 the free surface, and much elongated in the deeper portions, 

 the nuclei collecting at the ental poles of the cells. There is an 

 indication of close connection of the dorsal part of the walls of 

 the sac with the adjacent connective tissue, the epithelium cells 

 being apparently in process of transformation or degeneration, 

 losing their distinctness and becoming permeated with the sur- 

 rounding connective tissue elements. 



The chorda at this stage exhibits curvatures in the lateral 

 as well as the vertical plane. The latter curvature is obviously 

 due to the head flexure. The unaltered part extends to within 

 a short distance of the hypophysis, but the entire cephalic ex- 

 tremity has suffered a peculiar degeneration and reduction in 

 size, and is composed of closely massed nuclei bearing an un- 

 mistakable resemblance to the nuclei of the connective elements. 

 [Fig. I.] The end of the chorda is not only flexed and reduced 

 but irregular in outline. From a ventral protuberance of the de- 

 generated tip of the chorda a slender cord, resembling in struc- 

 ture the chordal sheath, passes ventrad and terminates in imme- 

 diate proximity to the sac with which, however, no direct con- 

 nection can now be traced. 



