46 AMERICAN HYDROIDS. 



the contiguous walls of the hooks when clasped as already described. While this solution was 

 not actually seen in any of the specimens described by me, it was found that the perisarc was 

 usually thinner in the region of contact than elsewhere. 



It must be remembered, moreover, that in the normal reproduction of most hydroids a solution 

 of the perisarc of the stem is effected, probably by chemical action, whenever a gouangium is 

 formed, 1 and therefore no new principle would have to be invoked to accomplish this end in the 

 case under discussion. 



In passing from below upward in the stem of a plumularian examined just before the appear- 

 ance of the gonangia, we find that the sex cells intergrade perfectly with the ordinary etidodermal 

 cells, many of which are themselves destined to become sex cells. The eudoderm then, in the 

 distal part of the stem, contains that which will ultimately become ova or spermatozoa, or it con- 

 tains what might be called the undifferentiated sex elements. A given colony of Aylaophenin is 

 always unisexual. That is, all the gonaugia contain sex cells of one kind, and both ova and sper- 

 matozoa are never found in one colony. 



Now it is evident that the hooking together of a male and a female colony by the upper parts 

 of their stems, accompanied by a dissolving of those portions of the perisarc which are in contact, 

 would leave only the thin ectoderm between the endodennal layers of the two colonies, and a 

 communication between the uudifferentiated sex cells would be an easy matter; for Weismann 

 found that the undifferentiated sex cells exhibited pronounced amu'boid movements, 2 and such 

 movements would, of course, greatly facilitate conjugation. The amu-boid cells observed by me 

 in the clasping hooks may be of significance in this connection. Not only did these cells exhibit 

 activity in sending forth pseudopodia, but they also moved bodily from place to place among the 

 surrounding cells. 



Since the foregoing material was published I have received a number of communications 

 concerning the matter of asexual reproduction. Several of my correspondents, as Professor 

 Yen-ill and Professor Bale, announce that they have observed lengthened processes such as 

 I found in Plumularia pinnata in several other species. Professor Verrill considers the process 

 as simply a modification of the well-known growth of new colonies by basal stolons. Of course 

 the liomology of hydrorhizae and hydrocladia has already been insisted upon in this work. The 

 stolon iferous reproduction, however, differs essentially from reproduction by basal stolons in 

 the fact that in the former case true hydrocladia with hydranths and sarcostyles are actually 

 metamorphosed into stolons, by which reproduction is effected, the new colony being entirely cut 

 off from the parent stem shortly after the development of the first hydranth. 



In regard to the possibility of conjugation among hydroids, the attitude of several correspond- 

 ents is well represented by that of Professor J. Playfair McMurrich, of the University of Michigan, 

 who writes: " Is not the process of conjugation a priori unlikely?" 



I frankly admit that the process of conjugation among hydroids is, a priori, very unlikely, and 

 it was in view of that fact that I have been most guarded in my language in the above paper, which 

 was intended to announce simply the evidence of the possibility of conjugation, giving the facts iu 

 full in order that readers might judge for themselves. There are considerations, however, which 

 increase the a priori possibility of this process. Part of these considerations have already been 

 given, but it might be well to mention in addition the fact that if any primitive methods are carried 

 over from the protozoa to the metazoa they would most likely be found in the hydroids or perhaps 

 in the sponges. Hydroids are exceedingly low in their organization and exhibit in several respects 

 the appearance of loosely aggregated assemblages of cells which are individually much like pro- 

 tozoa. The ectoderm cells, for example, are in many cases strikingly anneboid both iu appearance 

 and conduct, as are the endoderm cells in other cases, as, for example, the unditferentiated sex cells. 

 The wonderful facility with which lost parts can be replaced has astonished the world ever since 

 the classical researches of Trembly, and indicates an exceedingly undifierentiated condition of the 

 tissues involved. Not less remarkable is what might be called the interchangeability of parts 

 already insisted upon in this work, whereby one person or organ can be directly metamorphosed 

 into another. All of these facts indicate a high degree of plasticity on the part of the organism 



' Dr. August Winsmann, Die Kutstehuiig der Sexual/.ellen bei den Hydromeduseu, 1883, p. 182, 

 '' This fact was repeatedly observed by the present writer. 



