84 J- THOS. PATTERSON. 



cubated blastoderm just inside the lateral margins of the area 

 pellucida (Fig. i, A). Such injuries effect a fusion between the 

 ectoderm and entoderm, 1 and thus put up a barrier beyond which 



A B C D 



FIG. i. A-B are schemes for operating in Experiments I.-IV. respectively. 



the primitive streak mesoblast cannot pass in its peripheral 

 extension. 



The embryo, as it appears after forty-eight hours of incuba- 

 tion, is shown in Fig. 2. Instead of being circular in outline, as 

 under normal conditions, the vascular area is in the form of a 

 figure eight, the vena terminalis baying in on each side around 

 the obstructing injury-scars. It is easy to see how this result 

 was brought about. The lateral edges of the mesoblast, grow- 

 ing out from the primitive streak, came in contact with the two 

 injuries and at these points the mesoblast was prevented from 

 spreading farther, and so from coming in contact with the 

 germ-wall. There has been also a decided retardation in the 

 spreading of the mesoblast adjacent the injuries. Hence, the 

 transverse diameter of the vascular area, even at the widest 

 points, is only 8 mm., while the longitudinal diameter is 13 mm. 



The interesting point brought out by this experiment is made 

 evident when one turns to a study of a section taken across one 

 of the bays (Fig. 3, line A-B). The portion of the germ-wall, 

 which lies between the two sections of the vena terminalis, re- 

 mains undifferentiated (Fig. 4), that is, it has not given rise 

 to vascular mesoblast. It should be noted that it is from this 

 region of the germ-wall that the primitive streak mesoblast has 



1 The methods employed in this work have already been described in the 

 following papers : Patterson, J. T., " The Order of Appearance of the Anterior 

 Somites in the Chick," BIOL. BULL., Vol. 13; " Gastrulation in the Pigeon's 

 Egg A Morphological and Experimental Study" (in press), Journal of 

 Morphology. 



