THE MOSAIC STAGE OF DIFFERENTIATION 267 



regions of the gonad-rudiment, formed in different quantities, at 

 different rates, and with different capacities for diffusion. The 

 result is a bipotentiahty, and therefore a labiUty of differentiation 

 which makes the gonad especially interesting for a study of the 

 effects of external agencies upon morphogenesis. 



§12 



Problems of possibly a special nature are presented by experiments 

 of grafting portions of chick blastoderms on to the chorio-allantois 

 of other embryos. In general, it seems that the power of histo- 

 logical differentiation of a piece of blastoderm is conditioned by its 

 size, and by its age at the time of its isolation from the rest of the 

 blastoderm. An entire unincubated blastoderm, when grafted, will 

 show a degree of histological differentiation approximating to that 

 found in normal chick embryos which have been developing for the 

 same length of time. As mentioned above (p. 250), the morpho- 

 logical differentiation of such a piece may be very abnormal indeed.^ 



Small pieces, representing about one-fifth of the area of a whole 

 blastoderm, are restricted in their powers of histological differ- 

 entiation. Pieces of unincubated blastoderms grafted on to the 

 chorio-allantoic membrane will differentiate only into epidermis 

 and gut. Pieces cut from blastoderms incubated for 2 hours will, 

 in addition, differentiate into nervous tissue. After 4 hours' in- 

 cubation of the blastoderm, pieces cut from it will produce brain, 

 eye, cartilage, and muscle. After 10 hours, corium and feather- 

 buds are formed."^ It may also be mentioned that the earlier a piece 

 is isolated, the smaller are the organs which it forms. ^ 



It is thus apparent that the older a piece of tissue is at the time 

 of grafting, the better it will differentiate. This is especially well 

 shown in the case of the eye. A 4-hour piece will produce an eye 

 consisting of pigment cells only ; a 6-hour piece gives an eye differ- 

 entiated into pigment cells and retinal cells. After 8 hours, the 

 various layers of the retina are differentiated, while complete self- 

 differentiation of the eye is obtained from pieces cut from blasto- 

 derms that have been incubated for 33 hours. In the case of the 

 mesonephros, 4-hour pieces give secretory tubules, 6-hour pieces 



1 Murray and Selby, 1930. - Hoadley, 1926 a. 



^ Hoadley, 1929, 



