14 Dr. \V. Salensky on HdckeCs Gastraea Theory. 



sarcoJe-like body-mass, into which various small organisms 

 tiiul their way as nourishment and are there digested in the 

 same way as in the Infusoria. They have consequently a 

 mouth and the intestinal foundation [Dannanlage) ^ but are des- 

 titute of the stomachal cavity. We have no grounds for ex- 

 plaining the absence of the intestinal cavity in these animals 

 as a conse([uence of retrogressive metamorphosis*. 



These two circumstances (namely, l,the diversity of organi- 

 zation of the embryos of difierent animals at the time of the 

 formation of the intestinal cavity, and, 2, the acadous condition 

 of some Turbellaria) show quite sufiicicntly that we are not 

 in a position to derive the embryonal processes from the Gas- 

 trula, nor consequently to accept the Gastra'a as the stock- 

 form for the phy logenetic development of the JMetazoa. They 

 show that animals may possess the intestinal foundations, 

 without arriving at the formation of the intestinal and stoma- 

 chal cavities. From this it follows in general that we hardly 

 have any reason for assuming the presence of the stomachal 

 cavity in the stock-form of all Metazoa. 



This applies also to the two primary germ-lamellje, which 

 constitute the second important character of the (ra.'itrula-stagc. 

 Is the middle germ-lamella only developed when the two pri- 

 maiy germ-lamella?, the exoderm and entoderm, are already 

 at least perfectly formed, even if they do not together consti- 

 tute a Gasf )'ula-f orm? By no means. We can only say that 

 the middle lamella originates somewhat later than the other two 

 germ-lamellffi ; but in the majority of cases it originates long 

 before the stomachal cavity is formed, and it may even originate 

 at a time when the process of segmentation is not quite com- 

 pleted. After this differentiation of the first segmentation- 

 cells, the segmentation may still go on in all these layers of 

 cells. We know of such cases with the greatest certainty, from 

 investigations which have been carried on with perfect accu- 

 racy. One such instance Ave know in Euaxes, from the inves- 



• It might be objected that retrofrressive metamorpho.sis is bv no means 

 always dependent on parasitism, but that thi're are animals wliich pass a 

 free existence and yet undergo a retrogressive metamorphosis, e. g. the 

 males of the Rotatoria. But what is usually regarded as the retrogressive 

 metamorphosis of the male Rotatoria is really only an arrest of develop- 

 ment, and consists in the development of these animals remaining sta- 

 tionary at a certain stage, namely at that stage in which they poasess no 

 intestinal cavity, but only the foundation for the intestine. In the females 

 a cavity, the intestinal cavity, is formed in this foundation, but not in the 

 males. This mode of development presents essential diti'erences &om re- 

 trogressive metamorphosis, as in the latter the animals first show a higher 

 organization and afterwards lose it. ("See my " Hcitriige zur Entwickelung 

 des lirnchiouus urn-olaris," in Zeitschr. fiir wiss. Zool. lUl. xxii.) 



