ASYMMETRICAL DUPLICITY IN THE CHICK. 85 



a primitive-streak-like i-ucliment, and its longitudinal axis as 

 defined by its ''primitive groove" is, with reference to the con- 

 tinuation of the longitudinal axis of Rudiment a, bent to the 

 left at an angle of about 30°. 



Between the two embryonal formations there runs what appears 

 in the whole mount as a more darkly staining strand (PI. I. fig. 1) 

 which will be more minutely described in the sequel. 



The longitudinal axis of Rudiment a will be used as a basis 

 for the orientation of the whole blastoderm ; thus, when we speak 

 of the posterior end of the Rudiment /S we imply that portion 

 of it situated proximally or nearest to Rudiment o. 



B. Detailed description of the relations of the two Emhryonal 

 Formations and the structure of the lesser (Rudiment /3). 



Reference may now be made to the structure of the lesser 

 rtidiment, which is more properly described as " primitive streak- 

 like " than as a primitive streak. 



It will be seen from the figure that it consists of two main 

 parts, a rather diffuse and somewhat darkly staining oval area, 

 and the more deeply staining primitive streak proper (PI. I. fig. 1, 

 Rud. /3) with its more translucent primitive groove. 



The dimensions of the dark oval area, which the sections show 

 to consist of thickened mesoblast, are about 78/.< x 64/x, while the 

 length of the " primitive groove " is abovit 28ju. 



PI. II. fig. 2 represents a section of Rudiment ft in the region 

 of the primitive groove. It will be seen that it is quite com- 

 parable to the sections of a rather advanced primitive streak. 



The three primary germ-layers are well defined and separate 

 except in the region of the "primitive groove" {Fr.g.) where 

 the ectoderm and mesoderm are in continuity. One receives the 

 impression from these sections that there is, in this region, a well 

 marked proliferation of the median ectoderm as in the normal 

 primitive streak. 



Peripherally the mesoderm splits into somatic and splanchnic 

 layers on either side (PI. II. fig. 2, Som.mes., Sp.mes.), the coelom 

 thus formed being portions of the parietal cavity. 



The median un split mesoderm extends back from the region 

 of the " primitive groove " for some distance as will be seen from 

 PI. I. fig. 1 ; in section, it is seen to become free from the super- 

 imposed ectoderm, and this fact suggests the question as to 

 whether or not we might be justified in regarding it as of the 

 nature of a head-process. If we are so justified, it would become 

 necessary to regard Rudiment /3 as orientated in the reversed 

 direction to Rudiment n, i. e., the two embryonal formations 

 Avould be placed head to head (-• >> ■<— ) and not in the same 

 direction (— >■ — >). 



It may be noted that cases have been recorded of two opposing 

 embryonal formations placed upon a single germinal aiea. Such 

 a case was described by Mitrophanow (8), in which the two 



