16 Hjalmar Théel, 



are formed, irregularities in segmentation begin, but as I did not pay 

 special attention to segmentation I shall not attempt a description of 

 the process». 



Stadium with sixteen segments. 



This stage is accomplished at about two hours and 20 minutes 

 after the fecundation. First the four upper segments are divided by a 

 horizontal plane into four small cells or micromeres and four »large» 

 ones. Immediately after this segmentation or, though more seldom, be- 

 fore it is accomplished, the four lower segments are divided by a ver- 

 tical plane into eight almost equal spheres, which become arranged in 

 a more or less irregular manner in two rows. Sometimes it is diffi- 

 cult to decide whether they are placed in one or two rows, PI. II, fig. 

 25—26. 



Thus we find in this stage of development three different sizes 

 of segmentation spheres: at the upper pole four very small micromeres, 

 under these four »large» macromeres and at the lower pole eight some- 

 what »smaller» macromeres arranged in two more or less distinct rows. 

 Thus, the segmentation is now unequal. In order to prevent misunder- 

 standing I prefer to use in the following the terms : micromeres, »large» 

 macromeres and »smaller» macromeres. 



Stadium with thirty-two segments. 



Shortly after the preceding process of segmentation has been 

 finished, or at two hours and about 35 minutes after the fecundation, 

 the »large» macromeres begin to be divided by vertical planes into 

 eight equal spheres arranged in a row. The next division concerns the 

 »small» lower macromeres, each of them being parted by horizontal 

 planes into two equal segments, so as to present themselves in two 

 rows one above the other, eight in each. Lastlj^, 15 or 20 minutes 

 after the division of the »large» macromeres, the four micromeres at 

 the animal pole become parted by horizontal planes .into eight, four 

 of which are smaller, contain a clearer protoplasm and, as they lie in 

 the same plane, constitute the animal pole itself; the four remaining 

 ones have their place somewhat outside and below, PL II., fig. 27 — 29. 



In some cases I have observed that the »small» macromeres be- 

 gin this stage of segmentation, but this seems not to be a rule. As 

 ought to be understood from the above, the segmentation spheres pre- 

 sent themselves in four different sizes. 



