EARLY DEVELOPMENT OF THE LACERTILIA. 645 



the future posterior end of the embryo. At the narrow end an 

 invagination 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 epiblast 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 conversion 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 is anterior and median, and, although at first 

 independent of the epiblast, soon attaches itself to it, and forms 

 with it a kind of axis-cord. A fourth mass applied itself to the 

 walls of the sac formed by invagination. 



With reference to the very first developmental phenomena 

 my observations are confined to two stages during the segmenta- 

 tion 1 . In the earliest of these the segmentation was about half 

 completed, in the later one it was nearly over. My observations 

 on these stages bear out generally the statements of Kupffer and 

 Benecke. In the second of them the blastoderm was already 

 imperfectly divided into two layers a superficial epiblastic layer 

 formed of a single row of cells, and a layer below this several 

 rows deep. Below this layer fresh segments were obviously 

 being added to the blastoderm from the subjacent yolk. 



Between the second of these blastoderms and my next stage 

 there is a considerable gap. The medullary plate is just estab- 

 lished, and is marked by a shallow groove which becomes deeper 

 in front. A section through the embryo is represented in PL 29, 

 Series A, fig. I. In this figure there may be seen the thickened 

 medullary plate with a shallow medullary groove, below which 

 are two independent plates of mesoblast (me. p.}, one on each 

 side of the middle line, very imperfectly divided into somato- 

 pleuric and splanchnopleuric layers. Below the mesoblast is a 

 continuous layer of hypoblast (/ty.\ which -develops a rod-like 

 thickening along the axial line (c/i.}. This rod becomes in the 

 next stage the notochord. Although this embryo is not well 



1 For these two specimens, which were hardened in picric acid, I am indebted to 

 Dr Kleinenberg. 



