John Warren 3 
Fig. 4 is a section of an embryo of 13 mm. The yelum is again a 
little longer and its caudal layer is now distinctly thinner than its cephalic 
layer. The paraphysis is now a well-marked narrow diverticulum ex- 
tending dorsad from the paraphysal arch parallel to the velum. The 
paraphysal arch just cephalad to the opening of the paraphysis has been 
+ 
HIGeos 
Fic. 3. Embryo of 12 mm. Harvard Embryological Collection, Sagittal 
Series, No. 49, Section 58, X 63 diams. 
Fic. 4. Embryo of 183 mm. Harvard Embryological Collection, Sagittal 
Series, No. 598, Sections 71 and 75, * 63 diams. 
forced downward to a slight degree, as there is relatively more space 
between it and the ectoderm than in the previous figures. The epiphysis 
is about the same size as in Fig. 3, and its opening into the brain is 
clearly seen. 
Fig. 5 is a section of an embryo of 12.4 mm., which is, however, further 
advanced than that of Fig. 4. The velum, the post-velar arch, and the 
epiphysis are about the same, but the paraphysis is distinctly longer, and 
Fic. 5. Embryo of 12.4 mm. Harvard Embryological Collection, Sagittal 
Series, No. 675, Section 57, * 63 diams. 
Fie. 6. Same as Fig. 5, X about 120 diams. 
has become a narrow tube. The brain roof cephalad to it has descended 
still more into the cavity of the telencephalon and the opening of the 
paraphysis is much nearer the tip of the velum. Fig. 6 is the same 
section as Fig. 5, only drawn on a higher scale to show the histological 
details. The walls of the paraphysis and velum consist of a single layer 
of cells, with large oval nuclei and without very distinct cell boundaries. 
