398 Anton Dohrn 



scattered groups of neural-crest cells, found in some (not all) embryos 

 of eaiiier stages (17 mm.), bave been lost. 



In embryos of 21 mm some of the distai fibrils of the nerv 

 appear to bave united with migratory cells from tbe r. opbth. sup. 

 trigemini, a process in my opinion comparable witb that whicb 

 takes place in the distai giowth of the ociilomotorius. At least, 

 in embryos of 21 or 22 mm. the distai portion shows nuclei in relation 

 with the nerve fibres, whereas proximally no nuclei are seen. In 

 stili later stages the nerve has a distinctly cellular appearanoe 

 throughout its length. The nuclei are, however, seen in thin 

 sections to be peripheral in relation to the nerve fibres as in the 

 oculomotorius. The evidence of anastomosis of the fibres of the 

 trochlearis and the r. opbth. sup. trigemini I consider very doubt- 

 ful. During development the loose brush of fibres at the distai 

 termination of the trochlearis becomes united into a compact nerve 

 stem. It has therefore seemed to me, that the primary widely 

 spread brush of nerve fibres may be explained on the ground of 

 advantage gained in seeking the terminal organ, the m. obliquus 

 superior. 



The phenomena observed by me during the development of 

 the Trochlearis are seen to correspond very closely with those ob- 

 served by Miss Platt. To her, as to me, the trochlearis first 

 appears as a fibrillar process from the dorsal wall of the brain. 

 But while she interprets the evidence of cellular growth tovvard the 

 advancing end of the nerve as of morphological or phylogenetic 

 significance, I am unwilling to give it such Interpretation, since 

 I find that these nuclei bave nothing to do with the nerve proper. 

 In my opinion it is probable that they become couverted into the 

 nuclei of Schwann's sheath, an opinion which seems confirmed by 

 their peripheral position in relation to the nerve fibres. When the 

 only sections I possessed were of embryos killed with corrosive 

 sublimate acetic and stained with Carmine or hsematoxylin , the 

 evidence seemed to me confirmatory of the view ofFROKiEP, viz. that 

 the trochlearis is dififerentiated from mesenchymatous cells in situ. But 

 better methods of preparation bave tought me to distrust that 

 evidence, and the resulta appeared to me toc distinctly contra- 

 dictory to the later histological structure of the trochlearis to be 

 worthy of trust, or even of serious consideration.« 



Wie man sieht, hat Neal seine Auffassungen im Wesentlichen 

 auf den Befund bei Acanthias gegründet, und das bringt mich nun zur 



