270 Ross Granville Harrison 



impossible to keep such specimens alive after the yolk is gone. 

 Even when the brain and cranial ganglia are left intact so as not 

 to interfere with the normal movements of the mouth parts and 

 gills, the larvae soon succumb, owing to their inability to move about 

 and obtain food. During the course of my experiments in the 

 spring of 1906 a method was devised for providing the cordless, 

 and therefore paralyzed, larvae with nurses. This is accomplished 

 in the following manner: We start with embryos about 3 mm. 

 in length, in which no nerves are as yet differentiated. After 

 removal of the entire medullary cord caudal to the vagus region, 

 a small piece is cut off the side of the belly of the embryo and a 

 similar piece is taken from the opposite side of a normal embryo. 

 The wound surfaces of the two embryos are then brought together 

 and the embryos are held in place for an hour by means of pieces 

 of silver wire, as described by Born, after which time they are 

 permanently united. The further development of the pair takes 

 place normally except as regards the direct effect of the wound 

 healing, which brings about the formation of intestinal and vas- 

 cular anastomoses between the two. In this way the normal 

 component, which moves about and obtains food, is able to sus- 

 tain the pair for some time. When the yolk is about absorbed 

 the experiment is completed by transplanting to the nerveless 

 component a limb bud taken from a normal larva of the same 

 age or a little older. Such limbs contain, as already described,^" 

 the terminal twigs of nerves. The limb is grafted to the pos- 

 terior part of the trunk a little dorsal and cranial to the natural 

 hind limb, and in all cases was put on the free side of the body, /. e., 

 the side away from the nurse. The trunk region of the larva 

 remains nerveless except that the r. lateralis vagi is present. There 

 may be some extension of fibers from the nurse into the tissues of 

 the nerveless component immediately adjoining the former, but 

 only in one instance has a nerve been observed passing from the 

 normal component to the opposite side of the other. This nerve 

 passed under the notochord of the nerveless component after giv- 

 ing off twigs to the axial musculature and finally ended in the skin 



^"See p. 253. 



