204 



THE MOSAIC STAGE OF DIFFERENTIATION 



before they have united in the middle line, will, when explanted 

 singly, form vesicles of heart-tissue, and some (those from the left 

 side, see p. 77) may show pulsations.^ Heart-rudiments taken at 

 later stages, when they have united in the mid-ventral line, give 

 still more elaborate self-differentiation, showing sinus, auricle, 

 ventricle, and bulbus^ (fig. 98). 



be 



/I 



rc.l on I 



nl 



■,.,-1 



ami. 



A 



B 



Fig. 99 

 Mosaic development and self-differentiation of the chick eye-rudiment cultivated 

 in vitro. The eye-cup and lens were removed from an embryo incubated for 

 66 hours and cultured for 8 days in plasma with embryo extract. Histological 

 differentiation has proceeded at almost the normal rate, in spite of the fact that 

 the morphological differentiation of the structures is highly abnormal and that 

 they are subnormal in size. Histological differentiation is therefore independent 

 of morphological differentiation, and of the normal rate of cell multiplication. 

 A, Sectionof the whole explant. B,Section through the retina of a 17-day explant, 

 in which all the layers are normally developed, a.c. amacrine cells; b.c. bipolar 

 cells; ect. ectoderm; e.l.m. external limiting membrane; g.c. ganglion cell; 

 i.l.77i. internal limiting membrane; im.l. inner molecular layer; i.n.l. inner 

 nuclear layer; /. lens; M.f. Muller's fibres; o.ni.l. outer molecular layer; o.n.l. 

 outer nuclear layer ; p.cil. pars ciliaris. retinae •,p.l. pigment layer ip.op. pars optica 

 retinae. (From Strangeways and Fell, Proc. Roy. Sac. B, c, 1926.) 



In vitro cultivation of rudiments of presumptive regions has also 

 been practised with chick material. The optic cup (fig. 99), por- 



^ Goerttler, 1928. 



^ Stohr, 1924. 



