374 NATURAL SCIENCE. June, 



Mesenchyme. 



In a recent valuable memoir published by the Royal Society 

 (Philosophical Transactions, vol. clxxxvi., p. 163), Dr. Gadow gives the 

 result of an elaborate investigation made by Miss Abbott and himself 

 upon the evolution of the vertebral column in fishes. He takes occasion 

 to dismiss Hertwig's theory of the mesenchyme as a " gratuitous 

 hypothesis." While we are far from accepting it in the full sense 

 of Dr. Hertwig, we are unable to agree to Dr. Gadow's casual 

 dismissal of it. It seems to us that it is worth while to emphasise the 

 double nature of the blasts between the epiblast and hypoblast. In 

 Echinoderms and in Amphioxus, to take two examples, there is the 

 clearest possible distinction between the amoeboid wandering cells 

 that form the blood and connective tissues and the invaginations that 

 form the coelom. In the higher vertebrates the coelom is not 

 formed as a series of invaginations, and thus the distinctive character 

 of its mode of origin is lost. But still there seems to remain the 

 distinction between the set of cells that form the true ccelomic 

 system and the set of cells that form the skeletogenous layer and the 

 blood system. The origin of the two is more difficult to distinguish, 

 and we think that Dr. Gadow's paper is most valuable as clearing up 

 the exact mode of origin of the skeletogenous layer. But the 

 difference in behaviour of the cells of the skeletogenous layers and of 

 the ccelomic cells is equally striking. We do not think that the 

 mesenchyme is yet to be abandoned. 



The Vertebral Column of Fishes. 



Even after the most careful study of the paper, in the absence of 

 the necessary specimens, it is not easy to follow Dr. Gadow's 

 summary of his results. The protovertebrae of primitive mesodermal 

 segments divide into two parts. One of these, called the myotome 

 by Dr. Gadow, divides to form a myomere, or section of the trunk 

 musculature, and a part of the cutis. The other part forms a 

 sklerotome, which produces skleromeres or skeletal trunk segments. 

 The sklerotome or skeletogenous centre of each protovertebra con- 

 sists of a separate dorsal and ventral half. Each of these is a curved 

 S-shaped piece. In subsequent growth the dorsal half of each 

 sklerotome grows downwards and comes to lie behind the ventral half 

 of the sklerotome in front of it. Similarly, the ventral half of each 

 sklerotome grows upwards, and comes to lie in front of and below the 

 dorsal half of the sklerotome behind it. The acting units of the body 

 are formed by the union of the unequally numbered sklerotomic 

 halves in such a way that the dorsal half lies behind and above the 

 ventral half. From this mode of union it results that the myomere 

 or segment of the general trunk-musculature of the body comes into 

 relation, not with the skeletogenous element formed from the same 

 protovertebra, but with a compound element formed from the dorsal 



