MORPHOLOGY OF THE FOREBRAIN 457 



septi extends dorsally of the commissure, giving relations very 

 similar to those of the older embryo of fig. 54. The nucleus 

 lateralis septi also extends for a very short distance dorsally of 

 the interventricular foramen, but not so far as in the older embryo, 

 figs. 55 and 56. No cells from the septum extend farther back- 

 ward than the dorsal border of the foramen, the nucleus of the 

 commissura pallii posterior being as yet undeveloped, in correl- 

 ation with the absence of the commissure itself. 



Figs. 52 to 57 illustrate sections through the brains of older 

 embryos of Lacerta (about 36 mm. long when uncoiled, head 5 

 mm. long) from the Harvard Collection. Fig. 52 is a parasagit- 

 tal section through the brain of one of these specimens and upon 

 it is indicated the approximate plane of section of each of five 

 sections through another specimen. In this figure the precom- 

 missural body is seen to rise up in front of the interventricular 

 foramen and extend backward for a short distance above it. 

 The nucleus lateralis septi, separated from the nucleus medialis 

 by fimbria fibers, extends far dorsal and caudad into contact with 

 the margin of the cortex. The thalamus is related to the telen- 

 cephalon as in the turtles already described. 



Figs. 53 to 57 illustrate a series of sections taken in the planes 

 indicated on fig. 52. The primordium hippocampi is clearly 

 defined on the medial surface of the brain, being bounded by defi- 

 nite sulci at the level of fig. 54. The fissura limitans hippocampi 

 separates the precommissural body from the primordium hippo- 

 campi, and the latter is separated from the dorso-medial cortex 

 (cortex hippocampi) by the fissura arcuata. Attention is 

 especially called to the fact that the fissura arcuata lies within 

 the hippocampal formation, not below or above it, and to the 

 importance of this fact in determining its homology in vertebrates. 



The fibers of the commissura hippocampi are here divided, 

 one part crossing in the lamina terminalis as in turtles, here termed 

 the commissura pallii anterior (figs. 53 and 54), and another part 

 derived from the posterior pole crossing in the velum transversum 

 at the rostral border of the epithalamus (figs. 56 and 57). 



A special collection of cells belonging to the supra-foraminal 

 extension of the nucleus lateralis septi passes backward and far 



