EARLY DEVELOPMENT OF THE LACERTILIA. 425 



tinctly divided into epiblast find hypoblast.^ At the hind end of 

 the shield a somewhat triangular primitive streak is formed by 

 the fusion of the epiblast and hypoblast with a number of cells 

 between them, which are probably derived from the lower rows 

 of the segmentation cells. At the front end of the streak a 

 passage arises, open at both extremities, leading obliquely 

 forwards through the epiblast to the space below the hypo- 

 blast. The walls of the passage are formed of a layer of 

 columnar cells continuous both with epiblast and hypoblast. 

 In front of the primitive streak the body of the embryo 

 becomes first differentiated by the formation of a medullary 

 plate, and at the same time there grows out from the primitive 

 streak a layer of mesoblast, which spreads out in all directions 

 between the epiblast and hypoblast. In the axis of the embryo 

 the mesoblast plate is stated by Kupffer and Benecke to be con- 

 tinuous across the middle line, but this appears very improbable. 

 In a slightly later stage the medullary plate becomes marked by 

 a shallow groove, and the mesoblast of the embryo is then un- 

 doubtedly constituted of two lateral plates, one on each side of 

 the median line. In the median line the notochord arises as a 

 ridge- like thickening of the hypoblast which becomes very soon 

 quite separated from the hypoblast, except at the hind end, 

 where it is continued into the front wall of the neurenteric pas- 

 sage. It is interesting to notice the remarkable relation of the 

 notochord to the walls of the neurenteric passage. More or less 

 similar relations are also well marked in the case of the goose and 

 the fowl (Gasser),^ and support the conclusion deducible from 

 the lower forms of vertebrata, that the notochord is essentially 

 hypoblastic. 



The passage at the front end of the primitive streak forms the 

 posterior boundary of the medullary plate, though the medullary 

 groove is not at first continued back to it. The anterior wall of 

 this passage connects together the medullary plate and the noto- 

 chordal ridge of the hypoblast. In the succeeding stages the 

 medullary groove becomes continued back to the opening of the 

 passage, which then becomes enclosed in the medullary folds, 

 and forms a true neurenteric passage. It becomes narrowed as 

 the medullary folds finally unite to form the medullary canal, 

 and eventually disappears. 



1 conclude this paper with a concise statement of what 

 appears to me the probable nature of the much-disputed organ, 

 the primitive streak, and of the arguments in support of my 

 view. 



' This appears to me to take place before the formation of the eiii- 

 bryouic shield. 



2 Gasser, ' Der Primitivstreifeu bei Vogelembryonen,' Marburg, 1878. 



