W. G. 'VanName — Embryology of JEustylochiis. 2*75 



number, and may now be seen to be made up of microsomes, at least 

 so far as their outer portions are concerned. Toward the centers of 

 the asters they appear homogeneous and their structure cannot be 

 made out. They branch and connect with the cytoplasmic reticulum 

 and thus indirectly with the periphery of the egg. The centrosomes 

 (Figs. 5 and 36) have usually divided, so that there are now two 

 in each centrosphere. This division, which may however be delayed 

 till after the egg is laid, usually takes place in the direction of the 

 axis of the spindle or nearly so. This fact I shall have occasion to 

 speak of again, as it affects the process of formation of the sec- 

 ond polar spindle. The two derived centrosomes remain quite near 

 together, nevertheless the centrospheres usually become slightly 

 elongated in the direction in which they separate. The centro- 

 spheres are now bounded by a definite outline where the aster rays 

 take their origin. 



Fertilization. 



Eustylochus, like other polyclads, is hermaphroditic, but the eggs 

 of one individual are fertilized by the spermatozoa of another. The 

 spermatozoa may be injected into any part of the body, and find 

 their way to the eggs by their own activity, since they are able to 

 penetrate through the tissues. This process of hypodermic impreg- 

 nation is described by Lang (18) and other writers in other polyclads. 

 The spermatozoa are introduced into a puncture made in the epi- 

 dermis of the dorsal surface by the penis of another individual. 

 They are mixed with a quantity of thick mucus so as to form a 

 spermatophore of jelly-like consistency, and of fairly definite form 

 (Fig. 31). This mass of mucus and spermatozoa is not inserted 

 entirely below the epidermis, but the greater portion of it remains 

 projecting out of the animal as a white spherical mass, which may 

 be 0-6™"' or even more in diameter. Sometimes as many as four or 

 five of these spermatophores may be seen projecting from the same 

 individual at various points on the dorsal surface, but more often 

 toward the posterior than the anterior end. 



Living spermatozoa are 35-40 ^ long, and very slender and thread- 

 like. They taper at each end, but the anterior end is much more 

 slender, exceedingly sharply pointed, and moves in snake-like curves 

 or waves. These waves may travel in either direction, sometimes 

 with considerable rapidity, but with little effect in moving the 

 spermatozoon along in water, though no doubt efficient in penetrat- 

 ing the body-tissues. No definite demarcation between the active 



