36 AMEKICAN HYDKOIDS. 



DEVELOPMENT OF THE PLUMULAKID^. 1 



Origin of the sex eel In. All of the earlier writers supposed that the sexual products of the 

 hydroitls originated within the gonophores. Latterly, however, it has been demonstrated by 

 several authorities that this is not the case. A very complete and masterly discussion of this 

 subject is presented by Weismanu in his splendid monograph, Die Entstehuug der Sexualzellen 

 bie den Hydromeduseu, 18S3, a work which has already become classic and is a model of careful 

 and exact scientific research. The following account is practically a condensed translation of 

 Weismauu's description of the origin of the sex cells in I'lumularia echinulata. I had the pleasure, 

 while in Naples in 18!)o, of following to a certain extent in the footsteps of this master and verify- 

 ing in most details the account which he gives, working at the same place and with the same 

 species. Previous to this I had independently made some studies of the origin of the sex cells of 

 Plumularia pinnata and found that the main phenomena were identical with those presented by 

 P. echinulata. 



The male sex cells arise as follows: Upon examining a colony which bears gonangia on the 

 lower part of the stem it will be found that the joints above the upper gonangia contain groups of 

 germ cells situated in the endoderm and usually pressing against the stutzlamelle(fig.96). These sex 

 cells may be traced in their development by examining first the distal internodes of the stem and 

 working downward. At the top of the colony there, is no discernible difference between the indi- 

 vidual ectodermal and eudodermal cells. In the eighth or ninth internode from the top a number 

 of deeply stained cells will be seen in the endoderm. These cells, the "plasmareiche Zellen,"are 

 large and appear to be undergoing rapid cell division, the resulting cells being smaller, deeply 

 stained, and with distinct nuclei. 



These latter are the spermatoblasts. A little farther down these spermatoblasts are aggre- 

 gated into well-defined groups or masses called testicles, which lie in the endoderm awaiting their 

 migration into the gonangia, which will be described presently. 



Turning now to the development of the goiiangium, which is the next event in course of time; 

 the first change takes place in the ectoderm which intervenes between the mass of spermatoblasts 

 and the perisarc. Here the ectodermal cells, which are ordinarily small and polygonal, become 

 elongated in a horizontal direction, and eventually form a rounded cap of long cells which occupies 

 the whole thickness of the ectoderm, resting almost upon the mass of spermatoblasts, from which 

 it is separated by the stut/lamelle. This ectoderm is an important structure, designed to pene- 

 trate the thick perisarc of the stem, a performance which would apparently be impossible by any 

 purely mechanical process. In reality the process is not mechanical, but in all probability chem- 

 ical. According to Weismann, the cells of the cap have the power of excreting a substance 

 capable of dissolving the chitine, which is thus cut away, as it were, from in front of the growing 

 ends of the ectoderm cap which gradually advances through the thick perisarc of the stem 

 without exerting any mechanical pressure whatever. One fact maintained by Weismaun in this 

 connection is almost beyond comprehension, and that is that the youngest, inner, and most delicate 

 layer of perisarc is not dissolved by this action, but is pushed out through the remaining layers by 

 the ectoderm cap. It is therefore necessary to believe that the dissolving secretion passes through 

 one layer of chitine without injuring it, and then completely dissolves the remaining harder and 

 immt'iist'lj/ thicker layerx of the mime subxtance! 



However this may be, the penetration appears to be affected without obvious pressure, 

 and, in spite of the difficulties, Weisinann's idea of chemical solution seems the only conceivable 

 method. 



After the penetration of the perisarc by the ectodermal cap the endoderm again takes part in 

 the process. At first a prominence appears opposite the center of the mass of spermatoblasts 

 which presses against the Stutzlamelle and forms a concavity on the inner side of the endodermal 

 cap. The endodermal layer then pushes into the ectoderm cap, which, being liberated from the 

 confining perisarc of the stem, expands rapidly, the result being a hernia-like protrusion formed 

 of ectoderm, stut/lamelle, and endoderm, covered with a chitinons investment, a structure imme- 



1 Under tliis heading, as under Morphology, there will he no attempt to discuss matters which are not more or 

 less peculiar to the 1'lniiiularida 1 , reserving a more complete treatment of the subject for a future part of this work. 



