BOB1OB1 01 i MI: QBRN LAI IB fHBO 1 I' 1 



equivalent 10 tin- si-roiis ami tin* mucous I.-m-rs of BAKU. 

 after this (1853) A U.MAN introduced for the layers of the 



To IfntiM.-Hc- tin- names, which are now so much em{>l< /<////. 



and eiitniliTin ; subsequently use \\as al-o made of tlu-so for designat- 

 ing the embryonic l.-i\er>. 



lie germ-layer theory was promoted to a .-till greater degree by 



the Russian /oologist KOWALEVSKY, who made us acquainted in 

 numerous excellent detailed investigations with a profusion of 

 important facts concerning tne embryology of Worm*. CcelenteiM 

 Molluscs, Brachiopods, Tunica tes, and Arthropods. He produced 

 evidence that in all the Invertebrates which he investigated two 

 LMMin-1 ivers are formed at the beginning of development, and that 

 in almost all cases, when the process of cleavage is at an end, a 

 cellular sac arises, and that this, by the infolding of a part of the 

 wall, becomes converted into a double cup, the cavity of which, 

 enclosed by two germ-layers, communicates with the outside by 

 means of an opening. He succeeded in establishing the existence 

 of this very important cup-shaped larva (gastrula) in many branches 

 of the animal kingdom. 



In this connection should be mentioned the services of several 

 other embryologists, who at a still earlier period had observed in 

 isolated cases the cf//>-.v/m/W lrr<i <i//<? //.v i>rt;/ui !/ means of 

 I' untion. Rusmxi and E.EMAK had de-cribed the cup-shaped 

 larva of Amphibia, GEGENBAUR that of the Sagittae or arrow-worms, 

 M \\ SCHULTZE that of Petromyzon. 



Whereas KOWALEVSKY by his series of investigations enriched our 

 knowledge of material facts, HAECKEL first sought to utilise the 

 same for a general theory, since by the process of morphological 

 comparison he brought into association hitherto disconnected obser- 

 vations. Starting from the development and the anatomy of the 

 Sponges, he compared the layer-like structure of the embryos of all 

 animals with the layer-like structure of the Ccelenterates, and pro- 

 duced as the fruit of this study the celebrated gastrcea-theori/, which, 

 attacked on many sides at the time of its publication, has now 

 found in its essential substance general acceptance, and ha> gi\en 

 the impetus to numerous investigation*. II AK< KKI, showed that in 

 the development of the various classes of animals from the Sponges up 

 1 .in a single form of the germ makes its appearance, the gastrula, 

 which consists of two cell-layers, and that the two cell-layers of 

 tic various embryonic forms are comparable to one anotner or 

 homologous. The gastrula in its simplest condition presents, as 



