84 MR. NOEL TAYLER ON A UNIQUE CASE OF 



I have been unable to find any described instance of any 

 blastoderm closely resembling it, and it appeai-s of some interest 

 in connection with several morphogenetic questions, in particular 

 with the problem of the factors concerned in the early develop- 

 ment of the anterior portion of the embryo and also with that of 

 the localisation of the prospective embryo upon the blastoderm. 



It also has a beax-ing on at least two teratological problems, 

 the mode of origin of double monstrosity, and the nature of that 

 particular and little studied type of duplicity in which the two 

 components are unequally developed — the Duplicitas asymmetros 

 of Schwalbe. 



II. Descriptive. 

 The Morphology of Blastoderm E, and the embryonal formations. 



A. Genercd appearance of the Blastoderm and orientation. 



The blastoderm described in the following pages was very kindly 

 placed at my disposal some time ago by Prof. J. P. Hill, F.R.S., 

 to whom I wish to expi-ess my thanks for his assistance in reading 

 and criticising this paper. When the blastoderm came into my 

 hands it was in an unstained condition, and after being stained 

 with bor-ax carmine and examined as a whole mount it was cut 

 into serial sections for more detailed study. 



Unfortunately I am unable to state the time of incubation, of 

 which there is no record. 



In the first place I shall give a brief description of the gross 

 morphology of the blastoderm, and then after reference to the 

 structure of the lesser rudiment and to the relations of the two 

 embryonal centres as determined from a study of the sections, 

 shall proceed to a description of the morphology of the greater 

 embryonal rudiment. 



PI. I. fig. 1 is a reproduction of a photograph of the bl.istoderm, 

 which it will be seen is oval in shape, 12 mm. in length by 7 mm. 

 in breadth. 



Situated in the median longitudinal axis of the blastoderm is 

 the more completely developed formation, which will henceforth 

 be designated as Rudiment a or alternatively the greater or 

 major embryonal formation. It was found to possess 17 pairs of 

 mesoblastic somites, and is normally developed as regards its 

 posterior region. 



In PI. I. tig. 3 the abnormality of the anterior region, which 

 is chiefly evident in the position of the anterior intestinal poi'tal 

 and in the condition of the Fore-Brain, is apparent. 



In the anterior pi'olongation of the long axis of Rudiment a 

 and situated I'ather over 2 mm. from the anterior limit of its 

 Fore-Brain, is the lesser or minor embryonal formation which 

 will be designated in the sequel as Rudiment p (see PI. I. fig. 1). 



This lesser embryonal formation may be concisely described as 



