88 , MR. NOEL TAYLER ON A UNIQUE CASE OF 



proportions of tlie head, features of considerable theoretical 

 interest. PI. I. figs. 1 and 3 show well what we may briefly 

 characterise as the " shortness" of the head, and they further 

 emphasise the fact that in Rudiment a the brain does not play 

 the important part in moulding the general contours of the head, 

 which is the case in the noi-mal chick, and which is largely 

 consequent upon the considerable dilatation of the brain -vesicles. 



This abbreviation of the anterior region of Rudinient a is due 

 to two main causes, viz., the under-development of the Fore- 

 Brain and the condition of the heart and foi-e-gut. 



In the normal chick of 16-17 somites the anterior intestinal 

 portal is situated some distance posteriorly to the auditory vesicles. 

 In Rudiment a, on the contrary, the anterior intestinal portal 

 is anterior to these (PI. I. fig. 3 and PI. II. fig. 5, E 8, I. and 

 r.and.ves.). 



Now it is obvious that to describe the facts in this way is really 

 to beg the question, for one is assuming that the auditory vesicles 

 are normal in position. 



An alteration in the i-elative positions of auditory vesicles and 

 intestinal portal might result either fiom an abnormal develop- 

 ment of the bi^ain (for the auditory vesicles are normally placed 

 in relation to the neuromeres of the hind-brain) or from an actual 

 displacement of the anterior intestinal portal (i. e., an under or 

 over-development of the fore-gut), or it might be associated with 

 a combination of both these factors. 



By comparison of the measurements of Rudiment o with those 

 of normal embryos, it seems clear that in the case of the former 

 each of these factors plays a part in the production of the 

 condition described. 



Measurements of Normal Embryos. 



No. Ent. (Length (Length 



Designation. Somitts. Length. Ant. End — And. Ves.) Ant. End — A. LP.) 



Nl ]6 5"5 mm. 1"6 mm. 21 mm. 



N2 16 6 mm. 1-.5 mm. 2"1 mm. 



N3 15 6 mm. 1'4 mm, 1-9 mm. 



ISr4 17 6 mm. 1-4 mm. 2 mm. 



Mean of Measurements of Normal Embr3'os. 



5*9 mm. 1'47 mm. 2*02 mm. 



(nearly) 

 Measurements of Embryo a of Blastoderm E. 



16 6 mm. 1*3 mm. 1-2 mm. 



These measurements are reproduced above, and the}'' require 

 but little comment. The mean of the measurements taken from 

 the extreme anterior end to the plane of the auditory vesicles is 

 just under 1*5 mm., while the mean of the measurements from 

 the anterior end to the anterior intestinal portal is 2'02 mm. In 

 the ca.se of Rudiment o the former measurement is 1*3 mm., the 

 latter 1"2 mm. 



Hence the intestinal portal is situated no less than '8 mm. 

 more anteriorlv than in the normal embi'vo, wliile the lenfifth of 



