7 o THE POPULAR SCIENCE MONTHLY. 



The phenomena of yolk-segmentation, first observed by Prevost 

 and Dumas, are now known to be in some form or other invariably the 

 precursors of embryonic development ; while they reproduce, as the 

 first stages in the formation of the higher animals, the main and essen- 

 tial features in the life-history of the lowest forms. The " blastoderm," 

 as it is called, or first germ of the embryo in the egg, divides itself into 

 two layers, corresponding, as Huxley has shown, to the two layers into 

 which the body of the Ccelenterata may be divided. Not only so, but 

 most embryos at an early stage of development have the form of a 

 cup, the walls of which are formed by the two layers of the blasto- 

 derm. Kowalevsky was the first to show the prevalence of this em- 

 bryonic form, and subsequently Lankester and Haeckel put forward 

 the hypothesis that it was the embryonic repetition of an ancestral 

 type, from which all the higher forms are descended. The cavity of 

 the cup is supposed to be the stomach of this simple organism, and the 

 opening of the cup the mouth. The inner layer of the wall of the cup 

 constitutes the digestive membrane, and the outer the skin. To this 

 form Haeckel gave the name Gastrcea. It is perhaps doubtful whether 

 the theory of Lancaster and Haeckel can be accepted in precisely the 

 form they propounded it ; but it has had an important influence on the 

 progress of embryology. I can not quit the science of embryology 

 without alluding to the very admirable work on " Comparative Embry- 

 ology " by our new general secretary, Mr. Balfour, and also the " Ele- 

 ments of Embryology " which he had previously published in conjunc- 

 tion with Dr. M. Foster. 



In 1842 Steenstrup published his celebrated work on the " Alter- 

 nation of Generations," in which he showed that many species are rep- 

 resented by two perfectly distinct types or broods, differing in form, 

 structure, and habits ; that in one of them males are entirely wanting, 

 and that the reproduction is effected by fission, or by buds, which, how- 

 ever, are in some cases structurally indistinguishable from eggs. Steen- 

 strup's illustrations were mainly taken from marine or parasitic species, 

 of very great interest, but not generally familiar, excepting to natural- 

 ists. It has since been shown that the common Cynips, or gall-fly, is also 

 a case in point. It had long been known that in some genera belong- 

 ing to this group males are entirely wanting, and it has now been shown 

 by Bassett, and more thoroughly by Adler, that some of these species 

 are double-brooded ; the two broods having been considered as distinct 

 genera. Thus, an insect known as JVeuroterus lentieularis, of which 

 females only occur, produces the familiar oak-spangles so common on 

 the under sides of oak-leaves, from which emerge, not Neuroterus len- 

 ticularis, but an insect hitherto considered as a distinct species, belong- 

 ing even to a different genus {Spatheg aster baccarum). In Spathegas- 

 ter both sexes occur ; they produce the currant-like galls found on 

 oaks, and from these galls Neuroterus is again developed. So also the 

 King Charles oak-apples produce a species known as Teras terminalh, 



