I SEGMENTATION 5 



character," and that is unfortunately not justified by our knowledge 

 so far as it goes at present. 



SEGMENTATION 



The first important steps in the evolution of the unicellular 

 Zygote into the multicellular adult are seen in the process of 

 Segmentation which is, in fact, a process of mitotic cell division 

 showing special peculiarities in different groups of the Vertebra ta. 

 .During this process there appear in succession on the surface of the 

 egg grooves which gradually deepen and eventually divide the egg 

 incompletely or completely into dis- 

 tinct segments or Blastomeres. 

 Before entering into the details of 

 this process it will be convenient 

 to describe it in outline and define 

 the various technical terms used in 

 its description. 



The first phase of segmentation 

 is commonly marked by the appear- 

 ance of a superficial groove which 

 may conveniently be designated by 

 the letter a, passing through both 

 the upper and lower poles of the 

 egg. Such a groove or furrow is 

 termed meridional, as it marks a 

 great circle on the surface of the 

 egg corresponding to a meridian of 

 longitude on a terrestrial globe. 

 The single nucleus of the Zygote 

 meanwhile divides by mitosis — a 

 daughter nucleus passing into each 

 hemisphere. From the known facts, *°™i furrow; z, latitudinal furrow; 



„ „ -t i dional furrow ; v, vertical furrow. 



ol fertilization we have reason to 



believe that the Zygote nucleus contains exactly equivalent amounts 

 of chromatin from each of the two parents. In the process of mitosis 

 this maternal and paternal chromatin is again shared equally between 

 the two daughter nuclei. 



The first meridional furrow gradually deepens so that the egg 

 becomes completely divided into two blastomeres or segments each 

 representing a hemisphere of the Zygote. A second meridional 

 furrow (/?) now appears in a plane perpendicular to that of the first 

 and by the deepening of this the egg becomes divided into four equal 

 blastomeres. 



The next furrow to appear may be one running round the 

 equator of the egg (equatorial). In eggs, however, which are not 

 absolutely isolecithal — and this holds for all the lower Vertebrates — 



Fig. 2. — Diagram' to illustrate technical 

 terms used in describing the process of 

 segmentation. 



a.p, apical pole ; db.p, abapical pole ; e, equa- 

 m, men- 



