No. 3.] MORPHOLOGY OF THE ACT/N-OZOA. 305 



sion, though they are evidently thoroughly mature, and ready 

 for fertilization, after which segmentation begins without any 

 intermediate formation of polar globules. It seems almost cer- 

 tain that these bodies are formed before the extrusion of the 

 ova, possibly while they are still in the mesenteries. I have 

 endeavored in vain to observe their formation, but do not attrib- 

 ute my want of success to their non-existence. Haddon ('89) 

 has figured (PI. XXXVI, Fig. 10) a condition which he suggests 

 as representing the formation of a polar globule while the ovum 

 is still in the mesentery, but I cannot consider the formation of 

 polar globules before the setting free of the ovum, to be proved 

 by what he has figured, there being no evidence, such as the 

 existence of karyokinesis, to show that the body being extruded 

 from the ovum really represents a polar globule. It seems 

 more probable that the figure represents an artefact. 



The first indications of segmentation occurred in the ova of 

 Metridiiim about three-quarters of an hour after adding the 

 sperm to the water containing the ova, and consisted in the for- 

 mation of two slight elevations at one pole of the Q.gg (Fig. 2), 

 reminding one very much of what occurs in the ova of Cteno- 

 phores. A slight groove appears later between the elevations, 

 and, gradually deepening (Fig. 3), finally cuts the ovum into 

 two spherules. I did not observe the formation of the eleva- 

 tions in all cases, but it seems certain that the segmentation 

 furrow always starts at one pole of the ^gg in the typical Coe- 

 lenterate manner, — a method of segmentation, by the way, 

 which does not seem by any means to be confined to that 

 group, but to be of more widespread occurrence. 



The two spherules are usually slightly unequal in size (Fig. 4), 

 but the amount of the inequality may vary considerably, being 

 well marked in some cases, but hardly noticeable in others. 



When the segmentation furrow is completed, the spherules 

 are rounded, and in very slight contact with each other. Soon, 

 however, the contiguous faces flatten down (Fig. 4), and a re- 

 fusion of the spherules occurs. This phenomenon is very 

 marked in the four-celled stage. I have seen the spherules in 

 this stage become reduced by refusion to two, which, on their 

 part, underwent refusion, so that there was a return to an appar- 

 ently unsegmented ovum. I was not able, however, to watch 

 the further development of this ^g%, so cannot say that it was 



