486 Journal of Comparative Neurology and Psychology. 



to the infundibulum of man. The relative position of all these 

 structures is shown in Fig. 22, A and B, from a sagittal series of 

 an embryo in stage M. 



The study of sections can be controlled in these young stages by 

 the study of cleared whole embryos. Neal has given a most instruc- 

 tive series of figures from such cleared embryos and I can attest 

 the accuracy and faithfulness of these figures. If Figs. 7 to 11 of 

 ISTeaFs Plate 3 be examined, it will be seen that the optic vesicle 

 shifts from the "infundibulum" to a point in front of the anterior 

 head cavity. This agrees with what I have described above. I 

 have carefully studied these whole embryos with the Braus-Driiner 

 binocular and find that all the facts with regard to the form and 

 position of parts in the optic region of the brain derived from the 

 study of sections and models can be seen with perfect clearness in 

 the whole embryos. 



In Dohrn's papers on the mandibular and premandibular somites 

 (1904) the relations of neural plate and preoral entoderm discussed 

 in this section and the last are beautifully illustrated. Plates 1. 

 4, 6, 7, 9 and 11 show the early stages in the formation of the primi- 

 tive optic groove and terminal ridge and the relations of the preoral 

 entoderm and premandibular mesoderm derived from it. As is 

 well known, the anterior head cavity of Miss Piatt is found only in 

 the Squalidse and Dohrn does not regard it as a somite. I am forced 

 to believe, however, that he has not analyzed the conditions in 

 Squalus acanthias with sufiicient care, and that to this is to be attrib- 

 uted his attitude tow^ard the anterior head cavity as well as his failure 

 to recognize the terminal neural crest and the mesectoderm derived 

 from it. In his figures the primitive optic groove is labelled "infun- 

 dibulum," but it is perfectly clear to me that it is the groove related 

 to the optic vesicles. See PL 9, Fig. 9, where the groove marked 

 Inf. is the base of the optic stalk. In PI. 1, Fig. 15, the reference 

 line Ent. Zw. passes across the primitive optic groove at the front 

 and the true infundibular recess near the deep end of the hypophy- 

 sis. Compare Figs. 23, 24, and 25 of this paper. The terminal 

 ridge is especially clear in Dohrn's PI. 11, where early stages show 

 its form as well as the early stages of Amblystoma (see below). 



