16 Dr. W. Salensky on Héckel’s Gastrea Theory. 
the first processes of segmentation. Unfortunately this is 
difficult. The embryology of animals, and especially of in- 
vertebrate animals, has only for ashort 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 are 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 difticulty. And 
even when this difficulty is overcome, we have to do with 
contradictory statements by different observers; so that it is 
nearly impossible to draw general conclusions from the extant 
materials. 
Let us commence our examination of the process of segmen- 
tation and the formation of the germ-lamelle 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- 
lamellee, 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 arrive at the terminal form of the 
differentiation into two germ-lamelle, which form is perfectly 
similar to the Planula. 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, Lamellibranchiata, &c.), in the Vermes, Turbellaria 
(Keferstein, Knappert), in some Annelides (Huaxes and many 
Annelides observed by Claparéde and Mecznikoff), in several 
Crustacea, in which, 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 Beneden and 
Bessels, doc. czt.). 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 egg there is produced a two- or three-layered 
(as in Euazes), solid, generally ovoid or spherical body, which 
may also be characterized as a Planula, although m many 
cases it differs from the true two-layered Planula of the Coelen- 
terata by the presence of the three germ-lamelle. 
This process of differentiation of the germ-lamelle may im 
