212 INTERNAL FACTORS IV. i 



costae were very irregular. In these eggs there was, however, 

 only one stomodaeum, and only the normal number of canals. 



It appears, therefore, that the individual costae are definitely 

 related to the meridional divisions of the segmented egg, and 

 further that the material which is concerned in their formation 

 becomes located in the micromeres of the 16-celled stage. 



Driesch and Morgan have also shown that if the vegetative 

 hemisphere be taken away from the undivided egg, the animal 

 portion, though it segments normally, is able to produce neither 

 costae nor stomodaeum; it has, however, endodermal canals. Loss 

 of a small portion of the vegetative hemisphere, however, does 

 not interfere with subsequent normal development (Fig. 120 c, d}. 



On the other hand an egg which has been deprived of a lateral 

 portion divides, by three unequal divisions, into eight cells, all of 

 which form micromeres. Nevertheless, the larva has only four 

 five, or six costae, and some of these rudimentary, only three or 

 four canals, and the stomodaeum, as in half larvae, is oblique. 



As Driesch points out, the whole of the nucleus is here, and the 

 embryo is still defective. Differentiation thus depends on the 

 cytoplasm, but is quite independent of segmentation, which may 

 be perfectly normal without determining the formation of a 

 normal embryo. Driesch further argues that the observed defects 

 are due to lack of sufficient material merely, not to lack of any 

 preformed specific substance. Whether such substances are 

 already present in the unsegmented as they undoubtedly appear 

 to be in the segmented egg, it is perhaps impossible, on the 

 evidence, to decide, but it may be observed that the develop- 

 ment of perfect larvae from those eggs from which a portion of 

 the vegetative hemisphere has been detached does not necessarily 

 support the view advanced by Driesch. 



LITERATURE 



C.CHUN. Die Dissogonie,eine neue Form der geschlechtlichen Zeugung: 

 7. Zur Entwicklungsmechanik der Ctenophoren, Festschr. Lettckart, 1892. 



H. DRIESCH and T. H. MORGAN. Zur Analysis der ersten Entwick- 

 limgsstadien des Ctenophoreneies, Arch. Ent. Mech.ii, 1896. 



H. FISCHEL. Experimentelle Untersuchungen am Ctenophorenei, 1, 

 Arch. Ent. MecJi. vi, 1898 ; II, III, IV, ibid, vii, 1898. 



H. E. ZIEGLER. Experiinentelle Studien fiber die Zelltheilung : Die 

 Furchungszellen von Beroe ovata, Arch. Ent. Mech. vii, 1893. 



