396 E. V. COWDRY 



(4) The absence of controls. The different stages in the em- 

 bryological series have not as a rule been sufficienty accurately 

 characterized. In some contributions the only criterion has been 

 the number of hours of incubation, in others the length of the 

 embryos alone, so that it is frequently difficult to tell what stage 

 the investigator in question is describing. Furthermore, it has 

 been overlooked that synthetic comparisons between the struc- 

 tures of cells which do not occur in the same parts of the nervous 

 system of embryos of the same degree of differentiation are not 

 valid, since all regions of the ner\'ous system are never uniformly 

 developed. 



MATERIAL AND METHODS 



The material consists solely of chick embryos and is in part 

 summarised in table 1. The degree of specialization of the em- 

 bryos was determined by a consideration of: (1) the number of 

 mesodermic somites, (2) the period of incubation at 39°C., (3) 

 the length (measured before removal from the egg) and (4) a 

 comparison of their external morphology with the different stages 

 figured and described in Duval's Atlas d'embrylogie and in Kei- 

 bel and Abraham's Normentafel zur Entwicklungsgeschichte 

 des Huhnes (Gallus domesticus) . Reference was frequently made 

 to Lillie's text-book on the development of the chick, and to 

 Keibel's Normentafel zur Entwicklungsgeschichte des Kiebitzes. 



The methods include the observation of living tissue, with 

 and without the use of vital dyes; and of the impregnation and 

 staining of fixed material, imbedded in paraffin and in celloidin. 

 Each embryo was fixed, dehydrated and imbedded individually. 

 The technical methods employed are given in table 2. 



In the study of living tissue the portion of the nervous system 

 under investigation was isolated in warm salt soluton under the 

 binocular microscope and immersed in the staining fluid. The 

 stains were generally used in a concentration of 1 : 10,000 of 0.85 

 per cent sodium chloride solution. Preparations were examined 

 from time to time until the desired intensity of coloration had 

 been attained. 



