244 



WOUND HEALING IN EMBRYOS 



Sodium, Potassium, and Magnesium ions, hypertonicity, and alkalinity all cause lique- 

 faction and dispersion of the surface layer while Calcium, a slightly hypotonic medium, 

 and an intermediary pH, tend to counteract the effect of the former by binding and solidi- 

 fying the egg substances, thus favoring wound healing. Calcium, even in a concentration 

 of 1/100, 000, 000 is sufficient to aid the healing process and this amount may diffuse out 

 of the embryo from regions other than the wound area. The coated cells, in the wound 

 healing process, tend to spread over the uncoated ones. 



In an injured egg the edges of the coat at first retract, then 

 contract. Vital dye marks ore dragged out centripetally. 



A wound inflicted within the unsegmented area pulls into 

 its oibit neighbouring cells by the expansion oi the syncytial 

 coat. 



i^-' - 





/i^m^^-"- 



The contraction of on epithelial wound orients the adjacent 

 cells like the petals of a daisy. 



(Illustrations from Holtfreter: 1943. Jour. Exp. Zool. 93:251) 



The wound healing of later embryonic stages is even more remarkable than that of single 

 cells. More than half of the epidermis of a neurula may be peeled off and within a day or 

 so the remaining epidermis will spread to form a very thin covering epithelium over the 

 exposed or uncoated endo-mesoderm. Cell multiplication plays no significant role in 

 this healing process. Former cylindrical cells of a spreading epithelium nnay become 

 flattened out into a squamous epithelium. 



