ASYMMETRICAIi DUPLICITY IN THE CHICK. 



87 



the general proportions of head, the nervous system, the gut. the 

 vascular system, and coelom. 



Text-figure 1, 



Diagram of a longitudinal horizontal section. 



a. The general proportions of the anterior end and its relations to 

 the extra-emhryonal portions of the blastoderm. 



There is a complete absence of any anterior amniotic fold, as 

 is shown in the sections, PI. III. fig. 4, E 1 and E 2. The lateral 

 limiting sulci are fairly well marked (laf.l.sul.). There is no 

 head-fold and consequently no anterior limiting sulcus ; the ecto- 

 derm of the anterior end of the head of the embryo is continuous 

 with that of the blastoderm immediately in front of the head 

 without being recurved back under the latter, so as to delimit 

 it from the blastoderm as is normally the case at this stage of 

 development. 



Whilst the cranial flexure is, in a normal chick of 15-1 7 somites, 

 well marked, consisting of a bending of the antero-posterior axis 

 of the head, and there has also occurred its partial revolution 

 about the dorso-ventral longitudinal plane, PL I. fig. 3 indicates 

 at once that the antero-posterior axis of the head of Rudiment a 

 is perfectly straight, while the sections show that there is little 

 sign of any rotation (PI. III. fig. 4, E 1-E 5). The slight obliquity 

 of the dorso-ventral plane, which is seen in section E 4 (fig. 4), 

 is in the reverse direction to normal, and is no doubt the con- 

 sequence of the morphological asymmetry of the head which has 

 yet to be described. 



Reference must now be made to the general dimensions and 



