90 T. FU.TITA. 



zoon is often seen penetrating into the ovnm throngli ilie vitelline 

 membrane. 'J'he first polar body is soon followed l)y the second. Then 

 ensues the nnion of the male and the female pronnlei into a cleavage- 

 nucleus. By this time the spherules in the perivitelline space are being 

 gradually dissolved away, leaving behind a clear homogenous fluid, that 

 coagulates on application of acid reagents. 



After a short pause, the egg commences to segment. The first 

 segmentation plane is maridional, starting from the animal pole in a well 

 known way. The two blastsmeres thus formed are slightly unequal in 

 size. For a v\'hile, they are completely separated, but soon again apply 

 to each other, this time however so, that the center of the larger blasto- 

 mere comes to lie somewhat higher, e.i. nearer to the aiiimal pole of the 

 ovum, than that of the smaller. The second segmentation, also meri- 

 dional and at a right angle to the plane of the first, brings forth four 

 segments (A, B, C!, D, fig. 2), among which one (C) of the outcomes of 

 the original smaller blastomere sifts along toward the animal pole, 

 leaving its sister-segment (D) at her original position. The result is 

 that the three segments. A, B and C, are all on the same level, while 

 the segment 1) occupies a somewhat lower position. It is the latter, 

 that, later on, gives birth to mesoderm-cells. Moreover its position at 

 this early stage already indicat'^s the posterior end of future embi-yo. 

 The so-called cross furrow on the vegetative pole is formed by the 

 segments D and B. 



During the next following stages, four successive generations of 

 micromeres are l)udded off from each of the above-mentioned segments, 

 now to be called micromeres. Hereupon, the macromere D is entitled 

 to the name of entomesoderm and the remaining three macromeres may 

 be called entodermic macromeres. Synchronously with the formation of 

 the fourth generation of micromeres, each member of the third genera- 

 tion divides, thus giving rise to a fifth generation. At this stage there 

 are twenty micromeres and four macromeres, the relations of which may 

 be seen on fig. 8. Next comes in order the formation of a sixth gene- 

 ration of micromeres again from the third, followed bv that of a seventh 



