326 



GERMINAL ORGANIZATION INDUCTION PHENOMENA 



Fig. 17. Patterns of basophily due to RNA corpuscules, in the first week of the development 

 of the rat; an asterisk labels the animal pole; (a-d) seen obliquely, (e-i) along the primary 

 axis, (j-k) more or less along the primary axis. Big dots signify cortical mitochondria, small 

 dots mainly internal mitochondria; in (b-g) right, the lacunar appearance of the "ventral" 

 cytoplasm is suggested, (a) Young ovarian oocyte, from a primary graafian follicle; (b) 

 nearly ripe oocyte from a tertiary follicle; (c) ripe unfertilized egg, with second maturation 

 spindle; (d) fertilized egg aged about I2h.; (e) 2-cell stage (second day, from 30-4511. old) ; 

 (f) 3-cell stage (third day, about 56h.), with two recently formed blastomeres, the other 

 cell in anaphase; (g) 4-cell stage (about 6o-7oh.) where there is a distinct symmetry 

 pattern; (h) 8-cell stage (fourth day, 75-8oh.) with the blastomeres arranged nearly in 

 one plane and the same symmetry pattern; (i) young blastocyst (85-90 h.) with the first 

 appearance of a lecithocoele (L) and the distinction between an enveloping layer, formed 

 by the four large cells of (h), and an inner 8-cell mass; tiny RNA granules have just ap- 

 peared here; (j) somewhat elongated blastocyst of the fifth day, with a larger blastocoele 

 (or lecithocoele), a continuous enveloping layer of decreasing thickness, the inner-cell mass 

 intensely packed with RNA granules; (k) young cylinder stage (sixth day), still free in the 

 uterus, with the migrating endophyll (end.), while the rest of the enveloping layer be- 

 comes the trophoblast (tr.) ; (1) cylinder stage (eighth day) which has penetrated a uterine 

 gland, its trophoblastic part ahead, and is preparing to implant; increasing basophily 

 of the whole embryo. For the general appearance of further stages, see Fig. 37, e-i, p. 364. 

 RNA is then abundant in the mesoblast and in the induced parts of the ectoblast. From 



Dalcq, 1955. 



