24 F. M. BALFOUR. 



section tlirougli this embryo, and the sections belonging to Series 

 B illustrate the features of the hind end of the embryo and of the 

 primitive streak. 



As is shown in fig. 1, the notochord {ch.) has now throughout 

 the region of the embryo become separated from the subjacent 

 hypoblast, and the lateral plates of mesoblast are distinctly 

 divided into somatic and splanchnic layers. The medullary groove 

 is continued as a deepish groove up to the opening of the neuren- 

 teric passage, which thus forms a perforation in the floor of the 

 hinder end of the medullary groove {vide Series b, figs. 2, 3, 

 and 4). 



The passage itself is somewhat shorter than in the previous 

 stage, and the whole of it is shown in a single section (fig. 4) . 

 This section must either have been taken somewhat obliquely, or 

 else the passage have been exceptionally short in this embryo, 

 since in an older embryo it could not all be seen in one 

 section. 



The front wall of the passage is continuous with the notochord, 

 which for two sections or so in front remains attached to the hypo- 

 blast (figs. 2 and 3). Behind the perforation in the floor of the 

 medullary groove is placed the primitive streak (fig. 5), where all 

 the layers become fused together, as in the earlier stage. Into 

 this part a narrow diverticulum from the end of the medullary 

 groove is continued for a very short distance {vide fig. 5, mc). 



The general features of the stage will best be understood by an 

 examination of the diagrammatic longitudinal section, represented 

 in woodcut, fig. 1. In front is shown the amnion {am.), growing 

 over the head of the embryo. The notochord {ch.) is seen as an 

 independent cord for the greater part of the length of the embryo, 

 but falls into the hypoblast shortly in front of the neurenteric 

 passage. The neurenteric passage is shown at ne.^ and behind it 

 is shown the primitive streak. 



In a still older stage, represented in surface view on PI. Ill, 

 fig. c, medullary folds have nearly met above, but have not yet 

 united. The features of the passage from the neural groove to the 

 hypoblast are precisely the same in the embryo just described, 

 although the lumen of the passage has become som.ewhat narrower. 

 There is still a short primitive streak behind the embryo. 



The neurenteric passage persists but a very short time after 

 the complete closure of the medullary canal. It is in no way 

 connected with the allantois, as conjectured by Kupff'er and 

 Benecke, but the allantois is formed, as I have satisfied myself 

 by longitudinal sections of a later stage, in the manner already 

 described by Dobrynin, Gasser, and KoUiker for the bird and 

 mammal. 



The general results of Kupffer's and Benecke's observations, 



