76 



REG ULA R SEGMENT A TION. 



are usually known as segments or spheres. At the close of such 

 a simple segmentation the ovum becomes converted into a sphere 

 composed of segments of a uniform size. These segments usually 

 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 Coelenterata, some Nemertines, etc., 

 and in Mammals ; in which group the segmentation is not however 

 quite regular. All intermediate conditions between a large seg- 

 mentation 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 Sycaudra and the Cteno- 

 phora, the form of an axial perforation of the ovum open at both extre- 

 mities. 



When the process of regular segmentation is examined somewhat 

 more in detail it is found to follow as a rule a rather definite rhythm. 



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

 si), 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. 



The ovum is first divided in a plane which may be called vertical, 

 into two equal parts (fig. 39 A). This division is followed by a 

 second, also in a vertical plane, but at right angles to the first plane, 



