50 



NATURE 



[May 21, 1891 



eggs of Pallene were large, 0*25 mm., and well adapted 

 for investigation. After a minute description of the 

 early stages of development, the author considers that 

 from them there is little or no ground for a comparison 

 between the Pycnogonids and the Crustacea, certainly 

 not with any existing forms. The multipolar delamina- 

 tion of the endoderm in the Pycnogonids has no homo- 

 logue amongst the Crustacea, nor is there any special 

 similarity in the formations of the organs. There seems 

 to be no trace of gastrulation like that in the Crustacean 

 in the ontogeny of the group. And if there be reason for 

 rejecting a relationship between the Pantopod larva and 

 the Nauplius, and with Dohrn he believes that there is^ 

 then there remains nothing in common to the ontogeny 

 of the two groups. 



Nor are there any special affinities between the insects 

 and Pycnogonids ; but between these latter and Peripatus 

 a striking similarity is met with in the paired ventral 

 organs, both in the structure and position of these, but 

 for the present there is no proof forthcoming as to a real 

 homology of these bodies. The process of the formation 

 of the endoderm, as described by Heider and by Wheeler 

 in insects, shows a certain resemblance to multipolar 

 delamination ; but if it be such, it is a more complicated 

 form than is shown by the Pycnogonids. With these 

 two exceptions there would seem to be nothing else in 

 common in the ontogeny of the two groups. 



Lastly, as to a decision as to the relationship with the 

 Arachnids, or as to their being an independent phylum. 

 While Dohrn and Hoek ably maintain the latter, though 

 not agreeing as to the why in all details, yet the study of 

 the early stages of the embryology has brought to light 

 certain facts which lead the author of this memoir to 

 believe in a community of descent between the two. 

 The reasons for this belief are given in full detail, with 

 difficulty admitting of abbreviation. The Pycnogonids 

 form the endoderm by a process of multipolar delamina- 

 tion, which is shown in its simplest form in Phoxichilidium 

 and Tanystylum, and in a more modified condition in 

 Pallene. In no other group of the Triploblastica is a 

 similar phenomenon found except in the Arachnids. In 

 the spiders the process is not so well marked, but ii 

 Balfour's conception of the formation of the yolk nuclei 

 be correct, then a direct comparison may be made 

 between the two groups. The first trace of the embryo 

 to appear in Pallene is a round opaque area at the spot 

 where the stomodaeum invaginates. In Schimkewitsch's 

 recent account of the development of the spiders, he 

 shows that the primitive cumulus in them is the place 

 where the stomodaeum invaginates ; and in calling atten- 

 tion to the fact that the stomodaeum of spiders in its 

 earliest development is a triangular invagination, he 

 actually compares it with the triangular invagination of 

 the oesophagus of the Pycnogonids. It is also exceed- 

 ingly probable that the early formation of the body 

 cavity surrounded by mesoblast in the legs of spiders 

 has an exact parallel in Pallene and Phoxichilidium. 

 In both Arachnids and Pycnogonids there are well- 

 marked diverticula from the mid gut into the legs. In 

 both Arachnids and Pycnogonids the first pair of append- 

 ages are chelate, and in both this first pair is innervated 

 from the brain ; these facts alone, it will be remembered, 

 were considered by Balfour to indicate a relationship 

 NO. 1 125, VOL. 44] 



between the groups. Mr. Morgan was unable to find any 

 post-oral ganglia for Pallene, but the first pair of append- 

 ages arises on the sides of the stomodaeum and moves 

 forward later. In this respect, it compares closely with 

 the spiders, and the early innervation of this pair from 

 the brain itself may be regarded as a more abbreviated 

 condition than what was seen (by Balfour) in the spiders. 

 Metchnikofif's figures for Chelifer show the first pair of 

 appendages to arise above and on each side of the pro- 

 boscis-like upper lip, and if future investigation verifies 

 Metchnikofif's suggestion that this proboscis is homo- 

 logous, entirely or in part, to the proboscis of the Pycno- 

 gonids, as his figure seems to indicate, then does the 

 whole development of the Chelifer show remarkaibly close 

 resemblances to that of the Pycnogonids. The fourth 

 pair of ambulatory legs — the seventh pair of appendages 

 — has been a stumbling-block in the way of an Arachnid 

 relationship, and the attempts to solve the difticulty have 

 been many. Here, again, Balfour's suggestion that this 

 last segment and its appendages may represent the first 

 abdominal segment of the Arachnids is of value, as 

 we know that the embryos of spiders have rudimentary 

 appendages on the abdomen. In a second part of this 

 memoir the metamorphosis of Tanystylum is described, 

 and in a third part we have a very complete study of the 

 structure and development of the eyes of Pycnogonids 

 and a comparison with the Arachnid simple eyes, a com- 

 parison that seems to verify the relationship pointed out 

 in the first part of the memoir. E. P. W. 



A TEXT-BOOK OF CHEMISTRY BASED ON 

 THE PERIODIC SYSTEM. 



A System of Inorganic Chemistry. By William Ramsay, 

 Ph.D., F.R.S. Pp. 700. (London : J. and A. Churchill, 

 1891.) 



DURING the twenty-five years or so which have 

 elapsed since the recognition of the periodic law 

 of the chemical elements as a valid relationship, the 

 pronounced influence which it has exercised both on the 

 aspect and aims of chemical science cannot be ques- 

 tioned. Whether in the prediction of undiscovered ele- 

 ments, or as an indicator of needful research, especially 

 in the department of atomic weight estimations, it has 

 met with signal success. In connecting the physical 

 properties of the elements themselves and of their com- 

 pounds with atomic weight, it has opened up new fields 

 of investigation, and thrown fresh interest into old ones. 

 Properties so widely different as those measured by re- 

 fraction equivalent and breaking stress find an explana- 

 tion, nowadays, in the magnitudes of the atomic weights. 

 As a means of classification, too, the success of the 

 periodic arrangement has not been less striking. Indeed, 

 to its power as an instrument of classification it owes its 

 general acceptance in the first instance. When the ideas 

 of Avogadro had become recognized, and by their means 

 the old system of " equivalents " had been replaced by 

 the true atomic weights, then the periodic arrangement 

 resulted in a grouping of the elements so much in har- 

 mony with existing notions of their relationships, that 

 the far-reaching power of the generalization could no 

 longer be resisted. 



