Feb. 7, 1889] 



NATURE 



339 



segmentation is merobUistic. The ova of the West 

 Indian Peripatus, again, are yet smaller, and totally devoid 

 of yolk, and the segmentation is apparently complete. 

 In other words, the reduction in size of the ovum, due to 

 loss of yolk, which is still in process in /'. capcnsis, has 

 been achieved in the West Indian forms, and its effect 

 shows how easily the phylogenetic significance of seg- 

 mentation-types may be lessened. 



We can only mention a few of the more important facts 

 and generalizations of Mr. Sedgwick's memoir. Of 

 great interest is the observ^ation that the embryo, at any 

 rate at and until the gastrula stage, is a syncytium, i.e. 

 the various cells of which it is composed are only incom- 

 pletely marked off from one another, being connected by 

 radiating protoplasmic strands. Such a syncytial condition 

 the author regards as more primitive than the complete 

 separation of the segmentation-spheres from each other. 

 Another point in the early ontogeny is that no part of 

 the nucleus of the unsegmented ovum enters that central 

 portion of the syncytium which becomes differentiated 

 into endoderm ; and in this connection some recent obser- 

 vations of Hickson on segmentation in Millepora are of 

 value. The ovum in Millepora is almost devoid of yolk, 

 while those of the Hydrozoa generally possess a large 

 quantity ; and we venture to think that an earlier yolked 

 condition probably occurred in Millepora, though Mr. 

 Hickson has pronounced to the contrary. The segmen- 

 tation-nucleus breaks down into a number of deeply- 

 staining fiagments, which become scattered through the 

 cell, and eventually arrange themselves as the nuclei of 

 the blastula ; and it is at any rate possible that a similar 

 phenomenon occurs in the formation of the endoderm of 

 Peripatus, since Mr. Sedgwick describes (p. 26) " small 

 particles of a deeply-staining matter, which are neither 

 visible in the unsegmented ovum nor in the gastrula 

 stages, and which are not to be distinguished from 

 nuclear chromatin." From whatever source, amitotic 

 nuclei presently appear in the endodermal vacuolated 

 protoplasm, and the enteron is formed by the confluence 

 of these vacuoles. A solid gastrula is thus produced. 

 These facts lead Mr. Sedgwick to discuss the course of the 

 evolution of Metazoa from Protozoa. He pronounces in 

 favour of a "nucleated Infusorian-like animal, with pos- 

 sibly a mouth leading into a central vacuolated mass of 

 protoplasm," for the t<ansition-type, as against a colonial 

 Protozoan ; and declines to accept Metschnikoff's hollow 

 blastula as an even more primitive form than the solid 

 gastrula. 



The nephridia, the existence of which is one of the most 

 remarkable features of Peripatus, present two special 

 modifications, those of the third somite becoming the 

 salivary glands of the adult, and those of the twenty-first 

 functioning as generative ducts. The generative glands 

 themselves are formed as two continuous tubes from 

 the dorsal sections of somites 16-20 by a separation 

 from the ventral sections and absorption of the septa. 

 With reference to the coelom, Mr. Sedgwick comes to 

 several important conclusions. The mesoblastic bands 

 appear as a proliferation of nuclei at the lips of the 

 blastopore, which arrange themselves in groups round a 

 succession of cavities to form the future somites ; a 

 mode of connection generally taken to imply an ob- 

 scured enterocoele. This primary enterocoelic system 



of cavities is represented in the adult merely by 

 the generative glands and the nephridia ; the latter 

 are, as the author insists, not connected with, but actual 

 parts of, the coelom, and open each into a hitherto 

 undescribed vesicle in the leg, which at no period 

 communicates with the perivisceral space. Heart, peri- 

 cardium, and perivisceral cavity are the outcome of 

 spaces secondarily excavated in the mesoderm in con- 

 nection with a vascular system, and are best desig- 

 nated by Lankester's term hicmocoele. Such a hremo- 

 coele is characteristic of MoUusca and Arthropoda, 

 and Mr. Sedgwick's deductions tend to show that in 

 the latter group also the generative glands and ducts 

 and the excretory antennary glands are the sole remnants 

 of the true ccelom. 



In the second part, which deals with the genus from 

 a systematic stand-point, Mr. Sedgwick criticizes the 

 various forms hitherto described. He recognizes nine 

 good species, of which two are new : four from South 

 Africa, two from the Australian region, and three from 

 the Neotropical. The coloured plates which illustrate 

 this section are most creditable to the lithographers of 

 the Cambridge Scientific Instrument Company. In future 

 volumes of the " Studies " an exact reference to the place 

 where the original paper is to be found would often spare 

 trouble to the student of zoological literature. 



THE TEACHING OF CHEMISTRY. 

 The Fundamental Principles of Chemistry practically 

 taught by a New Method. By Robert Galloway, 

 M.R.I. A., F.C.S. (London : Longmans, Green, and 

 Co., 1888.) 



THE first thing that strikes one in taking up this 

 volume is that it requires cutting. This is a con- 

 siderable drawback to the student working from it, 

 especially in those cases where the description of an 

 experiment is continued on the next page, and to the 

 mere reader it involves a trouble that ought not to be 

 imposed upon him. But afar more serious fault is the 

 absence of even an attempt at an index. Whether the 

 author, the publisher, or the binder is to blame for this 

 omission is not obvious, but the fact remains that the 

 book is incomplete. 



The difficulty as to where the beginner shall begin 

 must have presented itself in some form or other to every 

 earnest teacher. Shall the facts come first .in their then 

 necessarily isolated condition ? or shall the student begin 

 with theories, making for himself so many mental pigeon- 

 holes into which the facts as they come may be put away 

 in an orderly manner .? 



The majority of teachers at the present day prefer ta 

 have something to classify before they attempt a classifi- 

 cation with their pupils, and in so doing we think they 

 adopt a perfectly sound and natural m:thod. The earnest 

 student is anxious to get on from the very first day of his 

 course, he craves to get hold of something tangible ; and 

 the teacher who treats him like an empty reservoir, that 

 is to be elaborately prepared and carefully tested as to 

 perfection of soundness before any water is admitted to 

 it, will ignominiously fail. 



Probably every teacher of chemistry has found difficulty 



