28 PROCEEDINGS OF THE AMERICAN ACADEMY 



namely, the precise origin of what His and others have called the 

 "parablast." The results of our investigation on this point enable 

 us to say that this layer, the origin and nature of which have been 

 the subject of so much controversy since the time of Lereboullet, cor- 

 responds to what is found in all vertebrate ova with discoidal cleav- 

 age, and lend strong support to the opinion that its mode of origin is in 

 all cases essentially the same. As these results appear to be irrecon- 

 cilable W'ith those recently published by Hoffmann, ^^ it is prope^ 

 that we should here state the methods by which they have been 

 reached. 



Methods. 



1. The successive phases of the cleavage, beginning with the mo- 

 ment of fecundation, were first of all followed many times over in the 

 living egg. Profile views and optical sections were obtained by tilt- 

 inof the microscope, the tube being inclined at different angles between 

 the vertical and horizontal positions, as recommended by Kingsley and 

 Conn. The eggs were confined in a live-box, and the light controlled 

 by the aid of Zeiss's illuminating apparatus (after Abbe). Two com- 

 plete series of vertical ojitical sections were obtained by the camera 

 lucida, one parallel with the longer, the other with the shorter axis of 

 the blastodisc. 



2. Mounted preparations of the blastodisc, in every stage of de- 

 velopment from the time when the pronuclei appear up to the time 

 when the germ-ring begins to form, were made in large numbers. I 

 have experimented with all the hardening reagents in common use, 

 and have failed to find any completely satisfactory method of preserv- 

 ing the vitellus. Even the germinal disc cannot be well preserved 

 by any of the ordinary hardening fluids. Kleineuberg's picro-sulphu- 

 ric acid, for instance, causes the cleavage products to swell, and 

 in many cases to become completely disorganized. The embryonic 

 stages can be hardened in chromic acid (one per cent), but the yolk 

 contracts considerably without becoming well hardened. The best 

 preparations of the cleavage stages have been obtained with osmic 

 acid followed by a modified form of Merkel's" fluid. This fluid, as 

 usfed by Dr. Eisig, consists of chromic acid (one fourth per cent) and 

 platinum chloride (one fourth per cent) mixed in equal parts. Thus 

 prepared it causes maceration of the embryonic portion of the egg. 

 By using a stronger chromic acid (one per cent) and combining it as 



18 Hoffmann, C. K. Zur Ontogenie tier Knoohenfisclie, Amsterdam, 18S1. 

 1* Merkel. Uebcr die Macula lutea des Menschen, Leipzig, 1870, p. 19. 



