500 



Special Vertebrate Organogenesis 



prospective conjunctival ectoderm in the ab- 

 sence of contact with the optic vesicle re- 

 mains pigmented and opaque like the sur- 

 rounding true epidermis. The ectoderm thus 

 appears to have a primordial capacity to 

 form epidermis. 



Inasmuch as the mesoderm is a relatively 

 thick layer, the upper surface of which is 

 in contact with skin ectoderm while its 

 lower surface is in close union with gut 

 entoderm and is the source of a variety of 

 organs and tissues, the question arises as 

 to what portion of the mesoderm contributes 

 to the formation of the dermis. Does the 



(the unsegmented mesoderm lateral to the 

 somites, within which the body cavity arises) 

 plus the closely united ectoderm and ento- 

 derm will also produce normal skin and 

 skin derivatives (feathers or hair, as the case 

 may be) when grafted to the embryonic 

 chick coelom as seen in Figure 192 (Rawles, 

 '47; Straus and Rawles, '53) or to the chorio- 

 allantoic membrane (Murray, '28). 



In view of the fact that it is assumed fre- 

 quently on the basis of morphological studies 

 (Engert, '00, and others) that the dermis, 

 not only of the dorsal and dorsolateral re- 

 gions but of the other body regions as well, 



P 



Fig. 192. Section through skin and do^^^^ feathers developing in intracoelomic graft of lateral plate isolated 

 from right side of 28-somite chick embryo at level of somites 22-25. Total age 10^/^ days. Compare with skin 

 and feather germs of normal. Fig. 193 C. Iron hematoxylin, Sfi X 75. 



dermis arise directly from mesodermal cells 

 which are everywhere in contact with and 

 subjacent to the skin ectoderm, or is its 

 origin restricted to the so-called "derma- 

 tome," one of the three arbitrary divisions 

 of the somite? An imequi vocal answer to this 

 old and controversial problem has been fur- 

 nished for the chick and the mouse by the 

 use of experimental techniques. Isolates of 

 early limb buds, taken prior to the entrance 

 of cells from the neural crest, and consist- 

 ing of somatic mesoderm covered by skin 

 ectoderm, will produce structurally normal 

 skin and skin derivatives, feathers and hair, 

 respectively, when grown in the coelom of 

 a chick embryo host (Hamburger, '39; 

 Rawles, '47). In the case of the chick, the skin 

 from such intracoelomic wing grafts has 

 even been transplanted to the back of newly 

 hatched host chicks, in which case it has 

 continued to grow normally and produce 

 typical wing plumage in normal succession 

 throughout the life span of the bird (Rawles. 

 "44). Moreover, isolates of the lateral plate 



is derived from the dermatome, the above 

 experimental results are of considerable in- 

 terest, for, under the conditions of the ex- 

 periments, the dermis found in the grafts 

 could not have been derived from derma- 

 tome material. Inasmuch as the neural crest 

 was also excluded from the grafted tissue, it 

 too may be ruled out as a contributing fac- 

 tor in the formation of the dermis of the 

 grafted regions, contrary to the suggestion 

 of Raven ('31, '36) for the urodele amphibi- 

 ans. It would appear from the experimental 

 evidence, then, that the dermis of the limbs, 

 flank, and ventral surface of the body is a 

 derivative of the mesoderm of the somato- 

 pleure. There is no doubt that the dermatome 

 of the somite contributes to the formation 

 of the dermis of the dorsal and dorsolateral 

 body regions. Grafts of somites, including 

 ectoderm and entoderm, without the adja- 

 cent lateral plate bear this out (Straus and 

 Rawles, '53). Furthermore, results obtained 

 from marking, with finely powdered blood 

 carbon, the mesoderm of various portions of 



