ii8 



NA TUBE 



[Nov. 29, 1883 



in the process, the priuiitively ectodermal germinal cells migrated 

 into tlie endoderm, and here v\ e find them mailing their first 

 appearance in all the Tubularina; bearing medusce or medu>oid 

 gonophores, in which they do not originate in the ectoderm of 

 the manubrium or in the endocodon. Most important is the fact 

 that in Podocoryne and Clava, and other forms, the male elements 

 have a different place of fi-st appearance from the female. In 

 Podocoryne the male germinal cells ari^e in the ancestral place, 

 the ectoderm of the manubrium ; the female, however, first 

 appear in the endoderm of the medusa bud. In Clava the male 

 elements originate in the. endocodon ; in the female they are first 

 detected in the endoderm of the gonophore stem. 



Here the phylogenetic shifting of the place cf first differentia- 

 tion of the germinal cells has operated cnly in one se.x or in one 

 mere than the other. In all such cases it is the place of first 

 differentiation of the female elements which has undergone 

 further shifting than that of the male, apparently because, under 

 similar circumstances, ov\ing to their more minute subdivision, 

 spermaries becon.e more easily and I'apicily ripened than ovaries. 

 In the case of EiidindriKm mcemosiim, already described, three 

 further stages of the shifting back of the place of origin of the 

 germinal cells ap|3ear to have been undtrgone by the female 

 stocks beyond those evidenced in Podocoryne. 



In s me forus, as in Cordylophora already described, the 

 entire long migration takes place entirely in the ectoderm, and 

 it is plain' that the shifting of the place of origin of the germinal 

 cells backwards from the gonophores has taken in different 

 forms two different lines of progress, one into the endoderm, the 

 other through the ectoderm only. It is a remarkable fact that 

 in no real medu.sa is the place of first appearance of the ger- 

 minal cells shifted further back than at most to the endoderm of 

 the gonophore. The difference of position of the generative 

 elements in the medu^ re of the Campanularinx is regarded by 

 the author as secondary, derived from a primitive disposition, as 

 in the Anthomedu-K, by phyletic shifting from the manubrium 

 to the radial canals, evidence in proof of which is adduced. 



A most intensely interesting section is that devoted to the : ub- 

 ject of the migration of the germinal cells. These cells seem 

 to be guided in their movements by an extraordinary instinct. 

 Every ovicell on setting out for its ti avals appears to have before 

 it a definite route to a particular gonophore, and to follow it 

 with certainty ; and, further, to be able to distinguish a young 

 hydranih bud from a young blastostyle bud, never entering the 

 one in error for the other. The migiations may be compared to 

 those of certain birds the young of which are 1 elieved by seme 

 ornithologists to find their way to their distant home without the 

 aid of any old birds who have already made the journey to 

 guide them. The author suggests that it must be the outcome 

 of an excessively fine sense of minute differences of pressure 

 which enables the t.vicells of Podocoryne, after they have bored 

 their way into the ectoderm, to arrange themselves in four longi- 

 tudinal rows in the intei radii of the manubrium, instead of form- 

 ing an even zone rtund it. No doubt, as he points out, the 

 same laws are at woik here which dettrmine the size, shape, 

 number, and sequerce of the cells in every organism ; but this 

 free mobility of these germinal cells in the Hydroida, with their 

 definite line of march and goal, is a new factor, to which there 

 seems to be no parallel known in other gnups, although migrat- 

 ing cells pursuing comparatively indefinite courses are known in 

 most Metazoa. As having a nearer resemblance to these move- 

 ments are cittd those of the mesoblast cells which are set free 

 from the blastoj hore of the gastrula larva of Echinoderms, and 

 which arrange themselves in regular order on the inner .surfaces of 

 its cavity. That there is no absolute difference between these 

 curious tissue-building migrations and ordinary growth follows 

 from the evident fact that they have arisen phylogenetically out 

 of the formation of organs \>y ordinary process of growth. 



Ihe question of the immediate origin of the primitive ger- 

 minal cells of the Hydroids is discussed in a most able summary 

 chapter of the utmost interest, but which it is impossible to do 

 justice to here. With regard to the relations of the elements to 

 the two layers, the conclusion is that in all the Hydromedusse, 

 including the Sip^honophora, the actual origin of the primitive 

 germinal cells is from ectoderm cells. In all cases in which the 

 first traces of the germinal cells can only be detected in the 

 endoderm, the parent primitive germinal cells have migrated out 

 of the ectoderm. This position is supported by two lines of 

 irgument, the one drawn from the comparison of the various 

 stages in the shifting of the place of origin of the germinal cells 

 exhibited in the various species of Hydromedusre, and especially 



in the two sexes of the same species, which points clearly to the 

 original and essential source of both sexual elements having lain 

 in the ectoderm, as is still the case in the primitive, hermaphro- 

 dite, freshwater Hydra ; whilst the other dwells on the circum- 

 stance that in all Hydroids in which the first appearance of the 

 germinal cells takes place in the endoderm, a satisfactory proof 

 of the endodermal origin of these cannot be brought forward. 

 Where they originate in the ectoderm their identity with young 

 ectoderm cells is obvious. When found in the endoderm, at the 

 bases of the peculiar flagellate cells composing this layer, they 

 have a similar appearance to the primitive germinal cells found 

 in the ectoderm, but no connection of gradation between them 

 and the endoderm cells can be detected, nor any subdivision ot 

 the endoderm cells tending to their production. 



Having arrived at the above conclusion, the author is led to 

 1 elieve, as already mentioned, that the division of the Ccelen- 

 terata in'o Endocarpas and Ectocarfse introduced by the brothers 

 Hertwig may very probably still hold good, the Hydi-omedusae, 

 with the Siphonophora and Ctenophora, being sprung from a 

 separate phylum of the primitive Coelenterates from that com- 

 prising the Anthozoa and Scyphomedusre. 



The work closes with a i-eference to the question of the 

 alternation of generations in the Hydromedus^. Now that the 

 ccenosarcal origin of the germinal cells is ]'roved in so many 

 instances, can the gonophores or medusa:-, the sexual cells of 

 which are formed in the ccenosarc of the hydranth or stem before 

 they themselves are begun to be developed, be. regarded as sexual 

 individuals ? It is obvious that it wr.uld lead only to confusion 

 if the old way of regarding the matter was upset. The past 

 history of the gonophores mut be taken into accoitnt, and the 

 fact that the sexual elements, though now developed at a greater 

 or less distance in many species, formerly undoubtedly originated 

 within the gono, hores. If an opposite view were adopted, the 

 absurd difficulty would arise that the male gonophores in some 

 species would have to be taken as sexual individuals and the 

 females in the same species as not. 



The author's discovery of the gradual phylogenitic shifting of the 

 place of origin of the sexual elements in Hydromedusas seems, a« he 

 points cut, to throw most happy light on the vexed controversy be- 

 tween Brooks and Salenskyasto the alternation of genei'ations in 

 the Sal I a?. The ovarium in the stolon of the solitai-y Salpa disco- 

 vered by Brooks doubtless belonged originally to the sexual 

 chain Salpte and has become shifted in order to hasten its matura- 

 tion into the stolon of the nurse, which is no more to be re- 

 garded as sexual because of its preparing an ovary for the buds 

 than are the jriiicijal hydranths of Eudtndrium racemosum 

 to be r< garded as such because they supply the ovicells to 

 the gonophores borne by the blastostyles. As in so many of 

 the Hydromedusa', the male elements of the sexual individuals 

 have undergone no corresponding shifting. The discrepancies 

 between the result; of the two observers probably depend on the 

 circumstance that the process of phylogenetic shifting has attained, 

 as in Hydromedusse, different stages of development in the various 

 species. The mode of reproduction of the Salpa; is still to be 

 regarded as a case of alternation of generation, even should 

 Salensky's w ell founded suspicion that the chain SalpEB are them- 

 selves able to produce a second ovary after the fir^t has been used 

 up prove invalid. 



The remarkable difterences in the development of the germinal 

 cells in nearly allied Hydromeduste seem to be paralleled to some 

 extent by the extraordinary condition in the early embryology of 

 the Salpre discovered by Salensky,' where the differences occur- 

 ring in the different species are so gi-eat and important that, as 

 he writes, "they hardly bear comparison with one another." 

 In all Salpfe the early segmentation of the ovum takes place as 

 u^uaI, but then " gonoblasts, ' cells derived from the epithelium 

 of the egg-folhcle, not sexually fertilised elements, suppress the 

 blastomeres, which atroi hy whibt the entire embryo is formed 

 from the gonoblasts with or without other unfertilised matter. 

 Salensky calls this extraordinary process, which is without 

 parallel in the rest of the animal 1 ingdom, "follicular budding." 

 Possibly some of the curious difterences as to the extent 10 

 which Ihe gonoblasts and parts of the ovary and oviduct enter 

 into the formation of the embryo in Salpse (Gymnogouje and 

 Thecogonre) may le hereafter explained on some such principle 

 as that of Prof. Weismann of "phylogenetic shifting." 



H. N. MOSELEY 



' Prof. W. Salensky, " Neue Untersuchungen uber die embryonale 

 Ent\\ icklung dtr Salpen." II. Th. Schluss, '* Mittheilungen aus der Zool. 

 Station zii Ne.ipl.," Bd. rv. Heft 3. 



