616 SUMMARY OF CURRENT RESEARCHES RELATING TO 



to the centre of tlie future blastoderm, and goes on most actively in 

 this region ; to this the author adds the rider that the point where 

 the most active segmentation commences corresponds to the future 

 posterior region of the blastoderm, and we can, therefore, early distin- 

 guish the front from the hind end. Like those of lower vertebrates 

 the ova of birds have a true segmentation cavity, which has the form 

 of a slitj is often difficult to recognize, and marks the point where 

 the ectodermal are separated from the subjacent elements ; as segmen- 

 tation extends more deeply it affects the yolk-layers which ought to 

 be considered as belonging to the white yolk ; but at a certain depth 

 this segmentation seems to stop ; it does not really do so, there is 

 only a modification of its rhythm ; the cavity formed by the furrows 

 is the subgerminal cavity which is produced from behind forwards, 

 and is the homologue of the primitive enteron of Batrachians, or 

 in other vs^ords, represents the gastrula-invagination of lower verte- 

 brates. 



After the formation of this subgerminal cavity a number of free 

 ■ nuclei are to be found in the yolk which forms its floor ; these arise 

 from nuclei which, during the formation of the cavity, had divided 

 into two ; of the halves one remained in one of the deeper spheres of 

 the blastoderm, and the other in the floor of the segmentation cavity. 

 A secondary segmentation appeared around these nuclei, which, at 

 first inactive, afterwards became very active ; the multiplication of 

 nuclei in the yolk gives rise to the production of the vitelline 

 endoderm. 



The blastoderm of the freshly laid egg is formed of two layers ; 

 the upper consists of a single row of cells, which form a distinct ecto- 

 derm ; the cells of the lower layer vary in size, are in the stage of 

 segmentation, and form an irregular mass from which arises both 

 endoderm and mesoderm; this may be called the primitive endo- 

 dermic mass. 



From the time when segmentation ends until the appearance of 

 the primitive groove, the edge of the blastoderm passes through three 

 stages ; it is at first raised into a ridge, and the ectoderm is continuous 

 with the endoderm ; the latter consists of several layers of cells and 

 forms the greater part of the swelling. The ectoderm then separates 

 from the primitive endoderm along the edges of the blastoderm, and, 

 while the ectoderm extends very far over the yolk, the margin of the 

 endoderm fuses with the yolk, to form an endodermo-vitelline enlarge- 

 ment ; as the yolk divides around each nucleus there appear large 

 cells which, by further division, increase the surface of the endoderm. 

 We next have a large vitelline layer (vitelline endoderm) with free 

 nuclei, and finally a layer of yolk without nuclei. 



The author finds that it is necessary to distinguish the axial plate 

 and the primitive line as two successive phases of one and the same 

 formation ; the former has the same constitution as the blastodermic 

 ridge. 



All along the axial plate the connections of the ectoderm with the 

 primitive endodermic mass exist from the moment when there first 

 appear the rudiments of this plate ; when its groove becomes more 



