vol. 3.j Robertson. — Embryonic Fission in Crisia. 131 



sometimes upon the tentacle sheath of a developing polypide, 

 sometimes lower down upon a septum, and sometimes free 

 in the mesenchyme which fills the interior of the tip. In 

 this last situation they frequently possess long processes which 

 suggest that they have an amoeboid motion. Their position, 

 however, is to be attributed not so much to their own movement 

 as to the fact that the tip has grown away from them, and has 

 left them suspended in the network of interior cells. PI. XIII, 

 Fig. 17, represents a section in which two such ova have been thus 

 left behind (or.) and which, like those embryos which reach only 

 a partial development, arc absorbed. Measurement shows that 

 the ova decrease in size as their distance from the growing point 

 increases, and in the lower zocecia no eggs are found, they having 

 gradually disappeared. 



A number of measurements of ova in various positions, e. </.. 

 those hi the ovaries, those on young buds or polypides, and those 

 free in the different portions of the internode, shows that much 

 variation in size occurs, but that these variations follow a regular 

 law. Thus a gradual growth can be traced from the very small 

 ova at the anterior edge of the tip, 5.4 /* in diameter, to older 

 ones measuring 10.8 /*, 14.4 /*, and 18 /*. A parallel growth of 

 the nucleus also occurs, those ova whose diameter is 10.8 f- pos- 

 sessing a nucleus of 7.2 m, while those whose diameter is 14.4 c- 

 and 18 ^ have a nucleus measuring 10.8 m. 



The eggs attached to buds or polypides are, as a rule, larger 

 upon the younger buds, and gradually diminish with the 

 development of the bud. Instances are found where the 

 ovum attached to the bud measures 21. G /* with a nucleus 10.8 m 

 in diameter. A frequent size upon young buds is 18 n, while 

 upon older buds and polypides it diminishes to 11.7 /«• and 10.8 

 H-, with nuclei varying in size from 9 /* to 7.2 a*. If the ovum 

 develops even partially (Fig. 16, emb.), the blastomeres of the 

 embryo, while large apparently, are smaller than the larger 

 ova. In the instance shown in Fig. 16, pd. 3, the boundaries 

 of the blastomeres are somewhat indistinct. One of them, how- 

 ever, measures 14.4 /*, while its nucleus is only 3.6 /*. Here, 

 although the size of the blastomere as a whole equals that of 

 some of the ova, the nucleus is much smaller. The outlines of 



ZOOL.-10 



