508 TEXT-BOOK OF EMBRYOLOGY. 



disappearance of that portion of the axone distal to the point of injury; it may 

 also lead to the disappearance of the entire neurone where regeneration is not 

 possible. Such an injury during development will not only cause a disappear- 

 ance of the whole neurone, but it may also lead to the disappearance of other 

 neurones forming links in the same functional pathway. Thus a develop- 

 mental defect involving the central area will not only lead to absence of the 

 pyramidal tract, but also to partial atrophy of the corresponding fillet bundles. 

 When one cerebellar hemisphere fails to develop, there results a correlated 

 defect in its centripetal and centrifugal pathways. The opposite inferior olive 

 is practically absent, as is also the central tegmental tract leading to that olive. 

 The pontile nuclei of the opposite side, the middle peduncle leading from them 

 to the affected cerebellar hemisphere, and the fibers in the pes which pass to 

 the pontile nuclei in question are likewise suppressed, and the superior 

 peduncle and red nucleus are absent or reduced. In this case it is evident that 

 the correlated atrophy affects at least two neurones in the pathways leading to 

 and from the cerebellum. This illustrates the far-reaching character of cor- 

 related developmental defects in the nervous system arising from the nature 

 of the connections between various portions of the system. 



PRACTICAL SUGGESTIONS. 



Very instructive pictures (surface views) of the neural tube (the brain vesicles, the spinal 

 cord, and the relation of the latter to the primitive streak) can be seen in chick embryos during 

 the second day of incubation. Remove the blastoderm from the egg, fix in Zenker's fluid, 

 stain in toto with borax-carmin, and mount in toto in xylol-damar. It is also interesting 

 to examine the blastoderm in the fresh condition. Even in chick embryos of later stages, 

 many of the general features of the developing nervous system can be seen in gross mounts 

 prepared as above. 



For details of structure, sections of embryos must be made. For this purpose mam- 

 malian embryos are usually available. The sections, if properly prepared, will serve a two- 

 fold purpose, viz.: the study of histological structure and of gross form. And since in such 

 complicated organs as the nervous system gross form can be studied to the best advantage 

 by means of reconstructions (see Appendix), serial sections should be prepared. Fix the 

 embryos (pig embryos, for example) in Bouin's fluid, cut serial transverse sections in paraffin, 

 stain with Weigert's iron haematoxylin and eosin, and mount in xylol-damar. Orth's fluid and 

 Zenker's fluid are also good fixatives. Hermann's fluid and Flemming's fluid, followed by 

 Heidenhain's hasmatoxylin stain, are also useful in the case of small embryos (not more 

 than 8 mm.). 



While general histological structure can be studied in sections prepared by the ordinary 

 technic (as above), other special methods are necessary for the study of certain structures. 

 Of the special methods, the most important are (i) the method of Golgi, (2) the method of 

 Cajal, and (3) the method of Weigert. All these have been more or less extensively modified. 



Method of Golgi. This has many modifications, but the method most in use is that known 

 as the Golgi rapid method. This consists in 



i. Fixing and hardening in potassium bichromate, 31/2 per cent., 4 vols. + osmic acid, 

 I per cent., i vol. These proportions may be varied. The time of hardening is very im- 



