GASTRULATION AND EARLIEST DEVELOPMENT 



189 



same time, but in one case the first predominates and in 

 the other the second prevails. As an example of the pre- 

 valence of polar polarity, 1 mentioned Nereis where the 

 macromeres (endoderm) are particularly large in comparison 

 with the ectomeres which lie over them in the form of a little 

 cap, while the bilateral polarity is only slightly manifested, the 

 cells of the rear side not being much larger than those 

 of the anterior side. On the other hand, this last condition 

 prevails very strongly in Scoloplos which accordingly can 

 serve as an example of the predominance of the bilateral pola- 

 rity. The cell 2d especially is of extraordinary size, 

 while the endoderm cells are not at all remarkable for 



a. b. c. 



Fig. 38. Diagrammatic representation of the 3 types of the eggs of 

 polychaetous Annelids 



I, I', III, 1st, 2nd^ and 3rd quartet of ectomeres. 

 ent. entomeres 



a. minute egg, b. egg with pronounced radial polarity, c. egg 

 with pronounced bilateral polarity. 

 The animal pole is indicated by a black dot. 



special bulk. Thus the endoderm area, as shown by the 

 diagram c of fig. 38, is displaced here to the ventral side 

 from the beginning, as is found in the same way in the 

 Qgg of the frog where, however, the cleavage process is 

 much more independent of the constitution of the Qgg. 

 In this way, I believe, is the dorsad displacement of the 

 endoderm area in the egg of lower Vertebrates to be 

 explained ^). 



') Thus the Amphibian egg in its structure corresponds to the 

 Scoloplos-type among Annelids. In a former article (19i6 b) I was 

 led to compare the egg of Rana esculenta to the type of Nereis. 

 This has not been confirmed by further investigations which showed 

 that there is no such great difference between \he e^gs of Rana fusca 

 and esculenta as I at first concluded from the study of more or less 

 abnormally developing eggs of the latter species. 



