CHAT". VI 



NEMERTINEA 



119 



The development has been worked out by C. B. Wilson (1900), and 

 later by E. B. Wilson (1903), but similar species exist in the 

 Mediterranean with very similar development, and have been worked 

 at by E. B. Wilson's pupils, Yatsu (1904) and Zeleny (1904). 



The egg of Cerebratulus lacteus whilst still in the ovary has a 

 loose glassy membrane which at one end is drawn out into a point 

 and thus offers a landmark in the egg ; after the egg has been laid 

 the spindle for the formation of the first polar body is formed, and 

 this causes a little protuberance at the opposite pole of the egg. 

 Unless fertilization supervenes the spindle is never completed, but 

 when fertilization takes place the first and then the second polar 

 bodies are nipped off and the loose glassy membrane is dissolved. 



The egg divides into four macromeres so exactly similar to one 



Fit;. 93. Two stages in the segmentation of the egg of Cerebratulus lacteus viewed 

 from the side. (After E. Wilson. ) 



71, polar bodies. A, 16-cell stage. B, 28-cell stage passing into 3'2-cell stage. 



another that it is quite impossible to distinguish an A, a B, a C, 

 and a I) segment, consequently the naming of the quadrants in the 

 segmenting .egg is an arbitrary matter. From these four macromeres 

 quartettes of micromeres are budded off" the first quartette dexio- 

 tropically, the second laeotropically, until no less than six quartettes 

 have been formed (Fig. 93). 



The exact fate of these quartettes is not described in detail by 

 Wilson. He remarks, however, that the endoderm extends to the 

 equator of the spherical embryo. From this we infer that, as usual, 

 the upper or ectodermic half of the segmented egg is constituted by 

 the three first quartettes of micromeres ; and that the fourth, fifth, 

 and sixth quartettes, together with the residual maeromeres, con- 

 stitute the endoderm. The residual macromeres are similar in 

 size and appearance to the last quartette of micromeres. 



In this way a hollow sphere of cells, the blastula, is formed which 

 is uniformly ciliated. Then one side of it becomes flattened and the 



