Developmental history of primary segments of the vertebrate head. 429 



cephalic axis terminates in the neuropore" (in : Verh. anat. Ges., 1893, 

 p. 101). (Author's translation.) 



As shown above (Figs. 10—15) in Salmo the infundibulum is a 

 ventro-caudad expansion of the second cephalic segment. The recessus 

 infundibuli is an expansion of the neural cavity of this segment. The 

 morphological front of the brain lies cephalad to this expansion and 

 is represented by segment 1. To this segment. the fibres of the ol- 

 factory nerves pass and the evidence would seem to be clear that the 

 latter constitutes the first pair of cranial nerves (contra Van Wijhe). 

 The optic nerves on the other hand are connected with segment 2 

 (see pages 407 and 412, Figs. 3 and 10), and thus become the second 

 pair of cranial nerves. 



In the chick the primary neural segmentation in the fore-brain 

 has disappeared when the optic nerves are produced and the infundi- 

 bulum evaginated. The chick therefore is a less favorable type than 

 the trout in which to study this relation. 



Neural Segments in the Chick. 



It is a noteworthy fact that no recent conflicting observations 

 are recorded regarding the segments of the medulla of the embryonic 

 chick. BÉKANECK, '87, Platt, '89, McClure, '90, Zimmermann, '91, 

 LocY, '95, all affirm that caudad to the cerebellum there are five 

 encephalic segments present in all chick embryos during the third 

 day of incubation. It is the region in front of the cerebellum that 

 constitutes the disputed territory. Miss Platt, '89, considers that 

 region made up of but one segment, the primary fore-brain and mid- 

 brain. The cerebellum constitutes the second segment, making a total 

 of 7 encephalomeres of equal morphological value in front of the 

 first protovertebra. She offers as evidence in support of this con- 

 clusion the presence of external transverse encephalic constrictions 

 and corresponding internal folds. 



McClure, '90, uses histological conditions as well as external 

 neural constrictions as criteria for encephalic segments. In this way 

 he finds at least 2 segments in the primary fore-brain and 2 in 

 the mid -brain of chicks in the 3. and 4. days of incubation. 

 The cerebellum constitutes his 5. neuromere which with the 5 of the 

 medulla make a total of at least 10 encephalic joints or segments. 

 I have studied McClure's figures and reviewed his contribution with 

 much care. If I understand him correctly his neuromeres 1 and 2 

 are restricted to the thalameucephalon. He does not mention the 



