21 



On the Eaely Development of the Laceetilia, together 

 with some Observations on the Kature and Eela- 

 TiONs of the Primitive Streak. By F. M. Balfour, 

 M.A., F.R.S., Fellow of Trinity College, Cam- 

 bridge. (With Plate III.) 



Till quite recently no observations were recorded on the early 

 developmental changes of the reptilian ovum. Not long ago 

 Professors Kupffer and Benecke published a preliminary note 

 on the early development of Lacerta agilis and Em^s Europea? 

 I have myself also been able to make some observations on the 

 embryo of Lacerta muralis. The number of my embryos has 

 been somewhat limited, and most of those which I have had have 

 been preserved in bichromate of potash, which has turned out a 

 far from satisfactory hardening reagent. In spite of these diffi- 

 culties I have been led on some points to very different results 

 from those of the German investigators, and to results which are 

 more in accordance with what we know of other Sauropsidan 

 types. I commence with a short account of the results of 

 Kupffer and Benecke. 



Segmentation takes place exactly as in birds, and the resulting 

 blastoderm, which is thickened at its edge, spreads rapidly over 

 the yolk. Shortly before the yolk is half enclosed a small 

 embryonic shield (area pellucida) makes its appearance in the 

 centre of the blastoderm, which has, in the meantime, become 

 divided into two layers. The upper of these is the epiblast, and 

 the lower the hypoblast. The embryonic shield is mainly distin- 

 guished from the remainder of the blastoderm by the more columnar 

 character of its constituent epiblast cells. It is somewhat pyri- 

 form in shape, the narrower end corresponding with the future 

 posterior end of the embryo. At the narrow end an invagina- 

 tion takes place, which gives rise to an open sac, the blind end of 

 which is directed forwards. The opening of this sac is regarded 

 by the authors as the blastopore. A linear thickening of epi- 

 blast arises in front of the blastopore, along the median line of 

 which the medullary groove soon appears. In the caudal region 

 the medullary folds spread out and enclose between them the 

 blastopore, behind which they soon meet again. On the con- 

 version of the medullary groove into a closed canal the blastopore 

 becomes obliterated. The mesoblast grows out from the lip of 

 the blastopore as four masses. Two of these are lateral : a third 



' * Die Erste Eatwicklungsvorgange am Ei der Reptilien/ Konigsberg, 



1878. 



