16 Dr. W. Salensky on HcickeVs Gastnva T/tcon/. 



t\\o tiist ])vocesses of scgnientation. Unfortunately tliis is 

 diliicnlt. The onibiyology of animals, and esi)ccially of in- 

 vertebrate animals, has only for a short time been the subject of 

 zealous investigation. During the last ten years we have 

 become acquainted, with so great a store of facts in this de- 

 partment of science, and these materials arc so scattered in 

 various natural-history periodicals, that a satisfactory colloca- 

 tion of all that has been published during this period on the 

 history of development is attended with much dithculty. And 

 even when this difficulty is overcome, wc have to do with 

 contradictory statements by ditferent observers ; so that it is 

 nearly impossible to draw general conclusions from tlie extant 

 materials. 



Let us commence our examination of the process of segmen- 

 tation and the formation of the germ-lamella} with those forms 

 in which the process of differentiation occurs earliest. Such 

 cases occur among the Rotatoria, in which, after the first binary 

 division of the egg-cell, the differentiation of the two germ- 

 lamellte, the animal and vegetative, is already indicated. In 

 each of these first two segmentation-cells, the further segmen- 

 tation takes place in a very different fashion. The smaller 

 cell continually divides and finally coats "the larger cell with 

 its derivatives ; and the larger cell also subsequently divides 

 into several cells. We an-ive at the terminal form of the 

 differentiation into two germ-lamella^, which form is perfectly 

 similar to the Plamda. Instances of the differentiation at a 

 somewhat later stage, after the segmentation has advanced to 

 four uniform segmentation-cells, are much more numerous. 

 They are apparently of very usual occurrence. They are met 

 with in the ]Mollusca (in the Opisthobranchiata, Prosobran- 

 chiata, Lamcllibranchiata, &c.), in the Vermes, Turbellaria 

 (Keferstcin, Knappert), in some Annelides {Euaxes and many 

 Annelides observed by ClaparJjde and !Meeznikoft'), in several 

 Crustacea, in wliich, however, very different modes of seg- 

 mentation may be observed in the different genera and even 

 species (Mecznikoff, ' Embryol. Studien an Insecten ' and 

 ' Entwickelung der Nebalia' [in Russian], Van Bcneden and 

 Bessels, loc. cit.). This later differentiation has the same re- 

 sult as that of the Rotatoria; the smaller cells grow round the 

 larger ones, which are richer in fat. As the result of the seg- 

 mentation of the Q.s;g there is produced a two- or three-layered 

 (as in ^'waxes), solid, generally ovoid or spherical body, which 

 may also be characterized as a Plamda, although in many 

 cases it differs from the true two-layered Planula of the Coelen- 

 terata by the presence of the three germ-lamellje. 



This process of differentiation of the germ-lamellffi may in 



