August 13, 1903] 



NATURE 



341 



Review, are no longer considered hypothetical, but 

 as real as dark stars. 



A new class of celestial bodies has thus been brought 

 under notice, and Miss Gierke does not sufficiently 

 appreciate its significance. This, however, is a matter 

 of opinion, but surely for the sake of historical com- 

 pleteness she might have mentioned that the associa- 

 tion of nebulae with new stars was first put forward 

 in the meteoritic hypothesis. She is careful to give 

 credit in most cases, but in connection with Nova 

 Persei no reference is made to the fact that Sir Norman 

 Lockyer first suggested in these columns that the dark 

 nebula existed before the star appeared. In the issue 

 of December 12, 1901, he wrote : — 



" It is impossible to think that the great nebula 

 which has now been photographed while the new star 

 is still in being did not exist there a few months ago ; 

 and it is important, further, to remark that the 

 nebulous matter already photographed in the region 

 round the Nova is very probably only a portion of the 

 actual amount of matter existing there, and that if 

 the disturbances continue, more of the remaining 

 portion may become visible." 



Here we have a definite statement of the pre- 

 existence of the dark cosmic matter in the neighbour- 

 hood of Nova Persei before the new star became 

 visible, but it has been overlooked by Miss Gierke. 

 This is to be regretted because, a few years hence, 

 astronomers will be just as interested in knowing how 

 the idea of dark nebulae passed from hypothesis to 

 demonstration as we are in Bessel's discernment of 

 the existence of dark companions of Sirius and Procyon 

 before these bodies came within the sphere of astro- 

 nomical discovery. 



One other point connected with Novae is worth 

 mention. In the description of the spectrum of Nova 

 Aurigae it is stated that " an exceptional feature was 

 the predominance of * green ' helium ; D, and the 

 rest of the lines belonging to the ' yellow ' set were 

 comparatively faint; while A 4922, \ 5016 and their 

 fundamental \ 6678, shone lustrously." An un- 

 necessary difficulty is raised in the attempt to account 

 for the appearance of these lines in the Nova spec- 

 trum ; for the first two lines mentioned were really 

 not due to helium, but were enhanced lines of iron 

 at A. 4924 and \ 5018. This identification does not rest 

 solely upon these two lines, for other enhanced lines 

 of iron appeared in the spectrum of the Nova. 



Other details upon which there are differences of 

 opinion might be mentioned, but no useful purpose 

 would h'i served by doing so. In directing attention 

 to the various points referred to in the foregoing re- 

 marks, the object has been to show that, though Miss 

 Gierke writes with exceptional facility and grace, she 

 is not an infallible guide, and has a tendency to works 

 of supererogation. Notwithstanding this, we do not 

 hesitate to say that, by writing the record of astro- 

 physics, she has done a great service to astronomers. 

 Her book makes it possible to obtain a view of the 

 chief fields in which astronomical inquiries are now 

 being carried on, and of the achievements which have 

 been reached. To readers interested in the progress 

 of knowledge relating to the sun, stars and nebulae, 

 whether they are laymen, or men of science so deeply 

 engrossed in other investigations that they have not 

 NO. 1763, VOL. 68] 



been able to keep in touch with astronomy, the book 

 will be a revelation. Those who are engaged in the 

 work of astrophysics will be saved many hours of 

 tedious research among scientific books and papers 

 by this chapter from the history of science. 



R. A. Gregory. 



THE GERMINAL LAYERS OF THE 

 VERTEBRATA. 

 Furchting und Keimhlattbildtitig bei Tarsius Spec- 

 trum. By A. A. W. Hubrecht. Pp. 115 + plates. 

 (Amsterdam : Mijller, 1902.) 



EMBRYOLOGISTS will certainly unite to con- 

 gratulate Prof. Hubrecht on the completion of 

 this memoir. To have obtained and figured a com- 

 plete series of developmental stages of any animal is 

 in itself no mean achievement, but when this animal 

 is one of the rarest of mammals, procurable only in 

 a distant quarter of the globe, we may well wonder 

 at the persevering patience which has succeeded in 

 overcoming difficulties which, to an ordinary worker, 

 would have been insurmountable. 



Tarsius has always been regarded as a member, 

 though a very aberrant member, of the Lemuroidea. 

 The embryological evidence which has now been 

 brought before us is practically conclusive in favour 

 of its removal from this suborder. The placentation 

 is most pronouncedly of the so-called " deciduate " 

 type, while the arrangement of the foetal membranes, 

 with the diminutive yolk-sac, rudimentary allantois, 

 and large extra-embryonic coelomic space, is identical 

 with that found in man and monkeys, but nowhere 

 else. 



The placenta, and the important changes leading 

 to the formation of the " Bauchstiel " — so long a 

 puzzle to human embryologists — have already been the 

 subjects of two publications by Prof. Hubrecht. In 

 the present treatise we are introduced to the processes 

 of maturation, fertilisation, segmentation, the his- 

 tology of the formation of the amnion, and, above all, 

 to the germinal layers. 



First to appear are the above-mentioned extra- 

 embryonic ccelom and the yolk-sac. The material for 

 the former springs from the posterior end of the 

 blastoderm. In continuity with it is formed the 

 primitive streak in the centre of which is the rudi- 

 mentary blastopore or neurenteric canal. The meso- 

 blast, however, is also formed from an anterior tract 

 of hypoblast (as frequently in Amniotes) and from a 

 peripheral ring (as described by the author in Sorex). 



These facts, admirably illustrated by a very complete 

 set of figures, form the basis for some very bold 

 speculations. The germ layers of the Vertebrata have 

 proved a stumbling-block to many an embryologist. 

 The solution of the problem here proposed (due origin- 

 ally to van Beneden, and first expounded in Oxford) 

 is one which cuts all the old ground from under our 

 feet. We are taken back, not to Amphioxus, or even 

 to an Annelid, but to a Gcelenterate, and asked to see 

 in the gastrovascular cavity and stomodaeum of this, 

 the latest ancestor of all the Vertebrates, the fore- 

 runners of the blastopore and notochord respectively. 

 Such a theory involves the assumption that the 



