DEVELOPMENT OF THE CHOROID PLEXUS 109 



In the 32 nun. embryo and in the 28 mm. embryo, the tela 

 chorioidea continues on to the epiphyseal evagination. A simi- 

 lar condition has been noted in the cat by Thompson ('09). 



c. Habenula. The character of the ridge running from the 

 anterior end of the habenula to the subthalamus in the 19 mm. 

 embryo (fig. 18, h.s-t.r.) is not apparent. It is probably only 

 a temporary fold mechanicall}' produced by inequalities in 

 development of the thalamic wall. It has entirely disappeared 

 in both older embryos. 



The ridge extending from the posterior end of the habenular 

 prominence in the 28 mm. embryo and the 32 mm. embryo 

 (figs. 20 and 22, f.r.) indicates the position of the fasciculus 

 retroflexus (Meynerti). 



d. Thalamus. In the 19 mm. embryo, only the anterior, 

 inferior part of the thalamic wall (fig. IS, th.t.) lying between 

 the sulcus limitans and the habenulo-subthalamic ridge is thick- 

 ened. In the 28 mm. embryo, the ridge has disappeared and 

 the anterior portion of the thalamic wall which lay above the 

 ridge is thickened (figs. 18 and 20, Ih. 2). This region probably 

 contains the principal nuclei of the thalamus. The posterior 

 extremity of the thalamus (figs. 18 and 20,) is still somewhat 

 flattened and thin in the 28 mm. embryo, but becomes very thick 

 in the 32 mm. embryo (fig. 22). 



e. Sulcus limitans. There is no difticulty in following the sul- 

 cus limitans. In the 32 mm. embryo under the posterior pole 

 of the thalamus, it is very deep. 



/. Hijpolhalamus. The hypothalamus in all these embryos 

 is of great antero-posterior extent. It will be noticed also that 

 the infundibulum, especially in the 19 mm. embrj'o, lies a con- 

 siderable distance back of the optic chiasm. It seems quite 

 probable that, as Johnston ('09) has remarked, in His' model 

 of the embryo CR of 13.6 mm., the recess marked infundibular 

 is really postoptic, and the real infundibular recess lies back 

 of it and is labelled tuber cinereum. Only in the 19 mm. em- 

 bryo is the subthalamus clearly separated externally from the 

 hypothalamus. 



