94 



It is more than likely, though I have taken no means to lind 

 out, that the median plane of the embryo coincides with the plane 

 of the first division of the ovum nucleus. But as will be seen from 

 the figures accompan^dng this paper, it is certainly incorrect to 

 speak of the next cleavage dividing the embryo anteriorly and 

 posteriori}^ If the position of the first plane be granted, the second 

 one marks rather the anterior end of the embryo or the position of 

 the head fold from which the stomodeum arises. 



The region of the polar cap which is to become ectoderm, and 

 that which is to be entoderm, can be identified in the fertilised egg 

 before segmentation begins. It might be affirmed therefore that 

 with respect to this region these two layers are pre -determined in 

 the cytoplasm. Not only so, but that the whole area is made 

 up of " Bezirke " or regions formmg a mosaic and pre-locahsed 

 as to the origin of particular organs. If this be true it must include 

 likewise a small region on each side of the mid-dorsal fine similarly 

 pre-determined to form a germ cell. Viewed in this wa}^ the polar 

 mass of cytoplasm is pre-locahsed only with regard to structures 

 which enter into the formation of the anterior end of the head. For 

 it is clear that during gastrulation the " Keimbezirke " give place 

 to " Randbezirke." Can it be said that in addition to a pre -deter- 

 mination of the elements forming the margin of the blastopore 

 the peripheral protoplasm which is incorporated during the growth 

 of the margin is also pre-localised with respect to the organs mto 

 which it may enter .? I have not Kopsch's * paper by me, but so 

 far as I remember his experiments do not refer to the peripheral 

 protoplasm. But one is almost justified in assuming that an injurj^ 

 to this layer would not have more than a mechanical effect upon 

 the growth of the postgastrula. If it be the fact that the mid plane 

 of the embryo is, for example, also pre-determined in the peripheral 

 layer a re-statement will be necessary mth regard to the growth of 

 the dorsal lip in embryos in which this layer has not yet been 

 described. There is nothing to be seen m the layer which would 

 lead one to say that in the dorsal region the layer would be separated 

 by the elements entering into the formation of the embryo proper, 

 and on the other side separated only by mesenchyme. I venture 

 to think therefore that we must accord to the i)eripheral cytoplasm 



* 1904, Unters. Gastr. u. Embryob. 



