Section IV., 1906. [ 21 ] Trans. R. S. C. 



III. — An Early Anadidymus of the Chick. 



By Professor Eamsay Wright, 



Biological Department, University of Toronto. 



Read May 23rd, 1906. 



The embryo which is descril^ed in the following pages was prepared 

 and sectioned in June, 1905, for class purposes but its abnormality 

 did not attract attention until it was brought into the laboratory. T 

 am, therefore, unable to figure the surface view, and so far have not 

 had leisure to model out its most interesting features. 



The series contains 200 sections of 15 microns in thickness, cor- 

 responding to a length of 3 mm. in the hardened condition. The egg 

 had been in the incubator for 24 hours, but, 10 somites having been 

 observed, it was marked as practically equivalent in age to Duval's 

 Gmbr3'o of 29 hours (No. 1, Fig. 89 and PL XYI). 



It was noted that the incubator was running at a temperature 

 somewhat higher than tbe normal, which may account not only for its 

 more rapid development, but also for its abnormality, as may be 

 inferred from Dareste (I^o. 2, page 121). 



Hertwig (No. 3: — Vol. I, p. 993) and others have remarked on the 

 rarity of cases of Anadidymus in Sauropsida in comparison with the 

 Ichthyopsida. This case is of particular interest, because, unlike Hoff- 

 mann's (No. 4, page 40) there appears to be no indication of a double 

 primitive streak, and, therefore, it is to be placed in the same category 

 ^-itli Dareste's embryo (Xo. 2, Plate 16, Figs. 5 and 6), and possibly 

 that of Mitrophanow (whose paper I have not been able to consult) 

 cited by Kaestner (No. 5, page 88). The occurrence of such a case 

 does not, in my opinion, invalidate the argument of Kaestner that 

 all such cases are primitively double (No. 6, page 141), because it 

 depends entirely upon the degree, locality and method of the inter- 

 ference of the two components, whether an organ shall appear double 

 or single. My figure of section 131 (Fig. 13) would not be suspected 

 to come from an embryo otherwise than normal, while the inspection 

 of section 126 (Fig. 12} at once shows that each half of it in reality 

 belongs to a different embryo. From this point, the interference 

 caudad has been more complete than cephalad, so that in the backward 

 growth of the primitive streak region (cf. Hertwig, No. 2, pp. 895 

 and 896) the embryo appears to be single. 



