Eoss Granville Harrison 201 



Description of the Experiments. 



The experiments described below were made upon the embryos of 

 Rana sylvatica, E. virescens and E. palustris, for the most part in the 

 spring of 1902. As already emphasized, it was necessary for the purposes 

 in view to work with embryos in which there were no traces of histologi- 

 cal differentiation in the nervous or muscular systems. It was found on 

 examination of serial sections of normal embryos that the oldest stage 

 which safely fulfills this requirement is when the tail bud is just begin- 

 ning to be perceptible. Sylvatica embryos (Fig. 1) are then about 3.7 

 mm., virescens about 2.25 mm., and palustris about 2.9 mm. in length, 

 although, owing to the considerable variation in the size of embryos of 

 the same species, these measurements are to be regarded merely as roughly 

 approximate. There are absolutely no nerve 

 fibers in the central nervous system of these /' ""'"" 



embryos and there are no traces of any peri- ' 

 pheral nerves. The tissue of the myotomes 

 consists of rounded cells somewhat flattened on 

 their sides. About ten somites are distinctlv "~" 



1 T ne rjM J. £ J.^ ■ ^ i ' FiG. 1. Embryo of R. sylvat- 



marked on:, i he rest oi the axial mesoderm ica. to show the stage of devei- 



. T opment used in the beg'inning' 



IS unsegmented. of the experiments. X 9hi. 



1. On the Effect of Removal of the Spinal Cord upon the Development 

 of the Axial Musculature. 



The embryos of E. palustris are somewhat better adapted for this 

 experiment than the other species. In the former the axis of the trunk 

 is straight, while in embryos of E. sylvatica the dorsal curvr^ture is 

 marked. Virescens embryos are more difficult to operate upon on account 

 of their smaller size. 



The embryo is laid on its side in a shallow dish lined with cork or paper 

 and containing fresh water or dilute (0.2 per cent) 

 ' salt solution. With a small sharp scalpel a narrow 

 \ strip extending from the region of the pronephros 

 to the tip of the tail is then cut off from the back 

 '"^l^l^^^"^ of the embryo (Fig. 2). This strip contains the 



Fig 2 Embryo of medullary tube, including the ganglion crest as far 

 j^eiy at^teT the iS'vii ^^ ^^ ^^ developed, the dorsal portion of the myo- 

 x^g^*.*^^ spinal cord. foiues and the unseg^nented mesoderm, and also 

 the dorsal fin fold. With some practice it is pos- 

 sible to cut just between the medullary tul)e and the notochord, leav- 

 ing the latter intact. One must count upon a number of failures, but 

 15 



