1893. SOME NEW BOOKS. 225 
germinal layer of equal value with epiblast and hypoblast. True, it 
appeared later in development, and its mode of origin was in dispute. 
Here there will be found a different conception. Hertwig divides 
Metazoa into animals with two germ layers and animals with four 
germ layers. In the latter, the two middle layers are both derived as 
definite organs from the inner layer; the one is the median noto- 
chord; the other, the paired sacs of the body cavity. In Amphioxus 
alone, this condition is perfectly definite; but the recent discovery' of 
segmental ducts, and nephridia in Amphioxus certainly will increase 
the content of morphologists in regarding Amphioxus as a represen- 
tative of the ancestral Chordata. In higher forms, the presence, 
actual or ancestral, of masses of food-yolk has so compressed the 
pouch-like evaginations from the primitive gut that they arise as solid 
outgrowths. 
Hertwig keeps by themselves these four primitive layers as 
‘‘ Epithelia,’ which serve for the limitation of the surfaces of the 
body. At the close of segmentation only one layer is present—the 
epithelium of the blastula. From this the other layers arise by in- 
foldings and outfoldings. But there is another element of develop- 
ment. In the Coelenterates and Echinoderms, when the two primary 
layers are established, there is found between them a supporting 
gelatinous layer. There wander into this from the primary layers 
cells most often starting from the point where these layers pass into 
each other. These immigrant cells lose their epithelial character 
and spread out as branched cells lke connective tissue corpuscles. 
This factor in development is called by Hertwig the ‘‘ mesenchyme,” 
and he finds it in the development of all higher forms as the origin of 
the connective tissues and the blood. 
Agreeably to the intention of the book as a text-book, this theory 
is laid down crisply and definitely, and it certainly is an extremely 
important advance on current ideas, although, for reasons that will 
readily occur to specialists, it may well be that the two middle layers 
and the mesenchyme do not exhaust that complex telescoping of for- 
gotten stages in the development of organs we call mesoblast. 
For the printing, illustrations, index, and references to literature, 
the highest praise is due. The translator has done his work well, but 
he is not to be congratulated on his translation of the word ‘“ anlage ”’ 
—a translation on which he plumes himself so much in the preface. 
‘*Fundament”’ is a clumsy word, and both the noun and adjective 
have meanings already in possession. As an English equivalent, 
‘‘ forecast’ is the best word, and for an adjective ‘‘incipient’’ is 
very convenient. 
Paco M. 
Diz ZELLE UND DIE GEWEBE. [THE CELL AND TISSUE.] By Dr. Oscar Hertwig. 
Pp. 300, with illustrations. Jena: Gustav Fischer, 1892. Price 8 marks. 
A Book treating of the structure of cells, written by Dr. Hertwig, and 
published by Herr Fischer, of Jena, is sure to be excellent in matter 
and in style. The volume before us is, on account of its very excel- 
lence, difficult to use as a text for a short discourse in this review. 
Merely to praise is apt to produce rather laborious reading. The only 
proper alternative would be to give a general account of the book; 
but this would occupy a little too much room; we shall therefore 
1 Since the publication of this edition. 
