OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 43 



circular, deeply marked line running round the periphery of the clear area. This line 

 appeared to be the commencement of the amniotic fold. Behind this line was a 

 narrow area in which there were a few dark spots, which marked apparently an early 

 stage of the formation of a vascular area. This area, in turn, merged into a crescentic 

 clear area which was limited on the opposite side by a short crescentic dark line 

 parallel to the first line. This line occupied the central part of the wider end of the 

 clear area, and lay across it, and it appeared to be the rudiment of a commencing head 

 fold. From the concavity of this fold a darker streak passed for a short distance 

 backwards towards the narrow part of the clear area, and this structure appeared to us 

 to be an indefinite stage in. the formation of the primitive streak. 



Specimen III. (fig. 2). This specimen resembled that of the chick of 22 hours. 

 The head fold was well formed, and behind it lay the medullary folds. These diverged 

 from one another at their posterior extremity, and behind them lay the remains of the 

 primitive streak stretching to the posterior end of the clear germinal area. The 

 primitive streak had the form of two narrow dark lines, enclosing between them a 

 narrow clear streak. 



Specimen IV. (fig. 3). Labelled as 3 days. The embryo was now 3 mm. in length 

 and presented well-formed medullary folds, a notochord, seven pairs of somites, and also 

 the remains of the primitive streak which was separated by a narrow clear area from the 

 posterior end of the notochord and extended for a distance of I mm. behind it. The 

 appearances were very similar to those of the chick at 25 hours. The medullary folds 

 were as yet ununited, but showed at the cephalic end evidence of distinct thickening, 

 while at the posterior ends they diverged from one another in a V-shaped manner. The 

 notochord could be traced as far forward as the cephalic enlargements (but this part of 

 the embryo was rather damaged), while posteriorly it terminated in a club-shaped 

 enlargement. 



Specimen V. .(fig. 4). The succeeding specimen, labelled as being 5 days, 

 showed a slightly more advanced stage of the same condition, closely similar to the 

 chick at 26 hours. 



The embryo measured 3 '5 mm. in length, and nine pairs of somites were present. 

 The posterior end of the notochord was enlarged and shaped like an Indian club, and 

 extended beyond the termination of the medullary folds, which diverged outwards on 

 either side of it. Behind this extremity was a clear area, and behind it again lay two 

 short parallel dark lines, the representatives of the primitive streak. 



The cephalic ends of the medullary folds were considerably enlarged, to form the 

 cerebral vesicles. These vesicles were not distinctly marked off from one another, but 

 the anterior part was bent forwards, to form the earliest cephalic flexure. In front of 

 this again was a narrow clear crescentic area the pro-amniotic region. 



Specimen VI. (fig. 5). Labelled 6 days. The next specimen was somewhat 

 larger, measuring 6 mm. in length. Twelve pairs of somites were present, and the 

 neural folds were larger, and had apparently united with one another at the fourth 



(ROY. soc. EDIN. TRANS., VOL. XLVH., 229.) 



