DEVELOPMENT OF EMBRYO OF FOWL. 49 



Fig. 8 represents the resulting embryo from the dorsal sur- 

 face ; the rudiments of the hind-limbs are apparently as well 

 developed as those of the anterior limbs, and the posterior por- 

 tion of the trunk is separated from the anterior by a sharp depres- 

 sion. The tail turns to the left, and tapers to an end beneath the 

 left hind-limb; part of its course is hidden by a fold of the soma- 

 topleure, as shown in the figure. The depression evidently repre- 

 sents the anterior limit of the injury. 



This embryo was sectioned. On examination it was found 

 that, although the region behind the depression has the usual 

 structure of the postanal region of the body, its neural tube has 

 no connection with that of the trunk, and the notochord is absent 

 in the anterior third. Mesoblastic somites occur in this region, 

 but the main part of its substance is made up of loose vascular 

 mesenchyme. 



It is plain, then, that this part has not regenerated in the sense 

 that it has grown out from the embryo ; and there remains only 

 the assumption that its parts have formed from embryonic tissue 

 remaining in this region after the cauterization. The absence of 

 the notochord throughout almost all of the tail shows that the 

 destruction due to cauterization was very extensive ; its absence 

 beneath the neural tube for a considerable distance shows, that 

 the neural tube has formed secondarily in this region ; because 

 any operation, that would destroy the notochord, would neces- 

 sarily destroy the neural tube. Farther, the relation of the 

 Wolffian ducts and allantois to the cloaca are abnormal, and the 

 hinder portion of the intestine has no connection with the anterior 

 portion ; all of which shows that there was originally complete 

 destruction of tissues through to the entoderm in the anterior 

 part of the cauterized area. There seems to have been in this 

 case a reorganization of the embryonic tissue of the caudal region. 



On the left side I find ten postbrachial ganglia back to the 

 region of the defect. Thus at least four segments of the normal 

 hind limb-region are included, viz., 7, 8, 9, 10. The last is small, 

 representing only a fraction of the normal ganglion. On the 

 right side there are eleven full-sized postbrachial ganglia. Thus 

 there are at least one and one half more metameres on the right 

 than on the left side, and this tallies with larger size of the right 

 hind limb and with the operation diagram. 



