176 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



is likely to be followed by an alteration in the stain, resulting in a faded 

 or yellowish appearance, and loss of good differentiation. 



Heidenhain's iron haernatoxylin stain was employed in the usual way 

 after fixation either in Zenker's fluid, or in a saturated aqueous solution 

 of corrosive sublimate to which had been added 1 per cent glacial 

 acetic acid. Besides giving its well-known sharp nuclear stain, iron 

 haernatoxylin differentiates clearly the primitive fibrils as soon as these 

 begin to appear in developing nerves. It is less favorable than vom 

 Iiath's mixture for the earliest stages of nerve formation, as it is not a 

 selective stain for the processes of the neuroblasts. After fixation in 

 Zenker's fluid or the^corrosive-acctio mixture, it is seen that these early 

 neuraxons appear frequently to approach, and to unite longitudinally 

 with one another, thus giving to the nerve the structure of a coarse 

 reticulum. 



Among the other general stains tried Brazilin and Delafield's haerna- 

 toxylin gave the most satisfactory results. Golgi impregnation and 

 intra-vitam staining with methylen-blue were attempted, but repeated 

 trials failed to produce the desired effect on the eye-muscle nerves of the 

 embryos. 



The method of van Gieson was used in advanced embryos for the pur- 

 pose of studying the first stages in the formation of the sheaths of 

 Schwann. By following Heidenhain's iron haernatoxylin with the van 

 Gieson mixture of acid fuchsin and picric acid, a good plasma stain was ob- 

 tained, which brought out distinctly the cytoplasmic processes of the cells 

 accompanying the nerve fibres, as well as those of the mesodermal cells. 



Serial sections of embryos of various ages were made in the following 

 planes : — 



1. Parasagittal. Such series gave nearly longitudinal sections of the 

 third and sixth nerves, and were best adapted to the study of their roots 

 and the cell migration from the neural tube, since the roots of these 

 nerves are spread out in a longitudinal but not in a transverse direction. 

 Obliquely longitudinal sections of the ophthalmic branch of the fifth 

 nerve were obtained in the series cut in parasagittal planes. 



2. Transverse to the longitudinal axis of the mid-brain. Owing to 

 the cephalic flexure, cutting in this plane gave longitudinal sections of 

 the third nerve, obliquely longitudinal sections of the sixth, and nearly 

 transverse sections of the ophthalmic branch of the fifth. 



3. Frontal to the mid-brain ; resulting in transverse sections of the 

 third nerve, obliquely transverse sections of the sixth, and nearly longi- 

 tudinal sections of the ophthalmic branch of the fifth. 



