vm ARTHROPOD A 179 



made to remove this membrane, the fluid mass flows out and the 

 egg is destroyed; so Reichenbach recommends the following pro- 

 cedure. The eggs are carefully removed from the parent and are 

 placed in water which is slowly heated to 70 C. ; then they are further 

 hardened by being immersed in a 2 per cent solution of bichromate 

 of potash for twenty-four hours ; then they are soaked for twenty- 

 four' hours in distilled water, which is often changed ; and finally, 

 they are transferred to 70 per cent to alcohol. By careful manipula- 

 tion with needles the thick " chorion " can now be removed and the 

 hardened egg escapes without injury. Reichenbach was accustomed 

 to remove the embryonic rudiment from the egg by means of a sharp 

 knife, then to stain it in picrocarmine, thoroughly dehydrate it and 

 mount it in Canada-balsam ; and he also studied the eggs by means 

 of sections cut parallel to and also transverse to the long axis of 

 the embryonic rudiment. 



A B 



FIG. 127. Two sections through the developing egg of Astacus. 



(After Morin, from Korschelt and Heider.) 



A, stage with few nuclei situated near the centre of the egg. B, stage where tho nuclei have reached 

 the surface, and the formation of the primary yolk pyramids has begun, n, nucleus ; y.p, primary 

 yolk pyramid. 



As will transpire immediately, there are many points of the 

 greatest interest in the development of the crayfish on which 

 Reichenbach's account throws insufficient light. If, as we hope, this 

 life -history should become the object of renewed investigation, 

 the method of imbedding in celloidin and paraffin, described in 

 Chapter II., would be of the greatest assistance in dealing with eggs 

 like those of the crayfish, which, owing to the number of yolk grains 

 they contain, are exceedingly brittle when hardened. 



The earliest stages in the development of Astacus were not seen 

 by Reichenbach, whose work begins with the stage when the nuclei 

 which result from the division of the zygote nucleus have reached the 

 surface of the egg, where they form a uniform layer all over its 

 surface. A Russian naturalist, Morin (1886), has, however, figured 

 earlier stages, and from him we learn that the zygote nucleus, as in 

 many other Arthropodan eggs, occupies at first a central position and 

 divides there ; and that the daughter nuclei are at first internal but 

 gradually migrate outwards till they reach the surface (Fig. 127). 



