GENESIS OF MAN. 39 



additional germ-layers out of the two original ones. The exact 

 mode of their development is still under discussion among embry- 

 ologists. Haeckel believes that the original exoderm and ento- 

 derm secrete each a new layer of cells from its inner surface ; that 

 is, from the surface of each which is contiguous to the other, so 

 that the two new layers lie against each other and separate the 

 primary by the thickness of both. It is, nevertheless, considered 

 that in the process the original constitution and identity of the 

 primary layers are destroyed, so that they have virtually resolved 

 themselves into four secondary germ-layers. The two outer layers, 

 however, now perform together the office of the original exoderm, 

 while the two inner ones take the place of the entoderm. This 

 division into four secondary germinative layers is the final division, 

 all the tissues, without exception, being formed out of these, as 

 they have in nearly every case been traced. 



The names assigned by Von Baer and by Haeckel to these sec- 

 ondary layers have reference to the functions which they are found 

 to perform. Being all German in their etymology, they are diffi- 

 cult to render into English. The following may answer as such 

 an imperfect version: — Numbering them from the outside, the 

 first is called by Von Baer the skin or dermal layer (Hautschickt), 

 and by Haeckel the dermo-sensory leaf or fold {Hantssinnesblatt). 

 The second is the muscular layer {Fleischschicht) of Von Baer, and 

 the dermo-fibrous leaf or fold (Haiitfaserblatt) of Haeckel. The 

 third is Von Baer's vascular layer (Gefiissschicht), and Haeckel's gas- 

 tro-fibrous leaf or fold {Darmfaserblatt). The fourth, or extreme 

 inner layer, Von Baer has denominated the mucous layer or mem- 

 brane {Schleimschicht), while Haeckel calls it the gastro-glandular 

 leaf or fold iDarmdrusenblatt). Space will admit of no further 

 following out of this interesting part of the history of embryonic 

 development. 



All worms are composed of these four secondary germ-layers, 

 the lowest possessing them in their greatest simplicity. The popu- 

 lar idea of a worm is an elongated creature, consisting of many 

 joints or rings {somites), but this is only a compound state of the 

 primitive worm, each ring or joint constituting, zoologically, a dis- 

 tinct individual, and possessing morphologically, if not physiologi- 

 cally, all the characters of one. The primitive worm has but one 

 joint. Among the lowest of the worms are the Tiirbellaria, which 



