3^4 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



controls pointed in favor of the passage of frog antigens into the Taricha tissue. 

 Recently, the study of mammalian embryos has called attention to a 

 relation between induction and alkaline phosphatases. As soon as the neurectoblast 

 is induced, it actively elaborates this enzyme (Fig. 37); the same holds true for 

 all further induced structures, such as neural crest, large and small placodes 



Fig. 37. Alkaline phosphatase in the early stages of the rat. This enzyme is not present 

 before the sixteen cell stage (a). It appears at this time in tiny granules around the nuclei 

 of the inner cell mass, and soon extends to the whole cytoplasm of these cells (b). (c) 

 blastocyst (5th day), with the immigration of the (negative) endophyll; (d) nidation stage 

 (6th day) with endophyll (negative) and still compact ectophyll; (e) young cylinder stage 

 (7th day) with the elongating embryonic region; (f) 2-layer stage (8th day) with the for- 

 mation of the amniotic cavity; the reaction fades appreciably at this stage; only the inner 

 part of the cells contains the enzyme ; (g) primitive streak stage (gth day) , in transverse 

 section, showing the same reaction in the ectophyll; (h) neurula stage (loth day) with the 

 neural plate strongly positive; (i) young embryo of the i ith day, with alk. phosphatase in 

 all induced structures and to some extent in the gut. A. c, amniotic cavity, in the amniogenic 

 region; Ac^, the same, in the embryogenic region; cer. pi., cerebral plate; g., gut; n. cr 

 VA^II, neural crest of the trigeminal and facial nerve; n. pi., neural plate; ot. pi., otic 

 placode; o.v., optic vesicle. After Mulnard's documents, 1955. 



