382 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



Pr. 



L.olf. p. 



R.olf. p 



R.c.d.s, 



'>_/ 



Fig. 50. An example of the effects of the in situ rotation of the central part of the medio- 

 dorsal zone, in the case of the slightly asymmetrical cut shown in Fig. 49, a. (A) The embryo 

 seen from its right side; there exists a dorsal hiatus {spina bifida) and the tail is curved dorsal- 

 wards; the cephalogenesis has taken place, but the head is distinctly atypical; (B) shows a 

 section through the middle level of the head; (C) is a graphic reconstruction of the main 

 organs as seen from their ventral side. The remarkable points are that ( i ) the notochord (circles) 

 deviates to the left, owing to the asymmetrical cut; (2) while the left notochord is uninter- 

 rupted to the end of the tail (which is suggested by an arrow), the right notochord is lacking 

 in the whole trunkal region, but reappears in the right wing of the tail (inset, below) ; (3) a 

 packet of myoblasts (crosses) is found inserted to the right of the chorda, and corresponds 

 to the X zone (Fig. 48, b) of presumptive somites ; (4) the right pronephros is strongly reduced, 

 with only one nephron and its nephrostom Na, while the left one is much larger, with two 

 nephrostoms (Ni, N2) ; a slight pronephritic vesicle has been transposed near the left 

 notochord (v.pr.), corresponding to the material z; (5) the right somitic material y (Figs. 

 49, a, b) has been combined with the left somites; (6) the anterior brain has induced two 

 nearly equal olfactory placodes, has formed two optic vesicles, the left one with a lens, 

 the "right" one nearly median, and, moreover, four slender optic vesicles (O.v. i, 2, 3, 4) ; 

 thus the inductive pattern of the acromerit has been considerably modified. Such operations 

 demonstrate that the rotated material dives directly towards the animal pole, without 

 turning around a blastoporal lip {kinematic regulation), and that the various areas of the 

 dorso-marginal zone are endowed with a high degree oi predisposition, usually blurred in 

 similar operations on Urodeles. 



Br-brain; Br. tr.-branchial tractus; Di.-diencephalon; End.-endoblast; H. -heart; L. br. 

 My. -left branchial myoblasts; L. e.-left eye; L. l.-left lens; L. olf. p. -left olfactory placode; 

 L.ot.-left otocyst; L. pr.-left pronephros; My (x) -transposed myoblasts (x area); N. ch.- 

 notochord; Na, Ni, N2- nephrostomes ; O.v. i, 2, 3, 4-supplementary right optic vesicles; 

 par.-paraphysis? ; Pr-prosencephalon; R. c.d. n. -right caudal notochord; R. c.d. s. -right 

 caudal spinal cord; R.e-right eye; R. olf. p. -right olfactory placode; R. ot. -right otocyst; 

 R.pr. -right pronephros; R. sp. -right spinal cord; Som.-somites; V. pr. (z)-pronephric 



vesicle. From Dalcq, 1941a. 



