THE SEGMENTATION OF THE OVUM. 



93 



a simple segmentation the ovum becomes converted into a sphere 

 composed of segments of a uniform size. These segments usu- 

 ally form a wall (fig. 39, E), one row of cells thick, round a central 

 cavity, which is known as the segmentation cavity or cavity 

 of Von Baer. Such a sphere is known as a blastosphere. The 

 central cavity usually appears very early in the segmentation, in 

 many cases when only four segments are present (fig. 39, B). 



In other instances, which however are rarer than those in 

 which a segmentation cavity is present, there is no trace of a 

 central cavity, and the sphere at the close of segmentation is 

 quite solid. In such instances the solid sphere is known as a 

 morula. It is found in some Sponges, many Ccelenterata, some 

 Nemertines, etc., and in Mammals ; in which group the segmen- 

 tation is not however quite regular. All intermediate conditions 

 between a large segmentation cavity, and a very minute central 

 cavity which may be surrounded by more than a single row of 

 cells have been described. 



The segmentation cavity has occasionally, as in Sycandra, the Cteno- 

 phora and Amphioxus, the form of an axial perforation of the ovum open at 



both extremities. 



E 



FIG. 39. THE SEGMENTATION OF AMPHIOXUS. (Copied from Kowalevsky.) 

 sg. segmentation cavity. A. Stage with two equal segments. B. Stage with four 

 equal segments. C. Stage after the four segments have become divided by an 

 equatorial furrow into eight equal segments. D. Stage in which a single layer of 

 cells encloses a central segmentation cavity. E. Somewhat older stage in optical 

 section. 



