CELL-DIVISION AND GROWTH 



393 



In the latter case, therefore, the body possesses an inherent polarity 

 which cannot be overturned by external conditions. A very curious 

 case is that of the earthworm, which has long been known to possess 

 a high regenerative capacity. If the posterior region of the worm 

 be cut off, a new tail is usually regenerated. If the same operation 

 be performed far forward in the anterior region, a new head is often 

 formed at the front end of the posterior piece. If, however, the sec- 

 tion be in the middle region the posterior piece sometimes regenerates 

 a head, but more usually a tail, as was long since shown by Spallanzani 

 and recently by Morgan ('99). Why such a blunder should be com- 

 mitted remains for the present quite unexplained. 



It remains to inquire more critically into the nature of the correla- 

 tion between growth and cell-division. In the growing tissues the 

 direction of the division-planes in the individual cells evidently stands 

 in a definite relation with the axes of growth in the body, as is espe- 

 cially clear in the case of rapidly elongating structures (apical buds, 

 teloblasts, and the like), where the division-planes are predominantly 

 transverse to the axis of elongation. Which of these is the primary 

 factor, the direction of general growth or the direction of the division- 

 planes } This question is a difficult one to answer, for the two phe- 

 nomena are often too closely related to be disentangled. As far as 

 the plants are concerned, however, it has been conclusively shown by 

 Hofmeister, De Bary, and Sachs that tJie groivth of tJic mass is tlic 

 primary factor ; for the characteristic mode of growth is often shown 

 by the growing mass before it splits up into cells, and the form of 

 cell-division adapts itself to that of the mass : *' Die Pflanze bildet 

 Zellen, nicht die Zelle bildet Pflanzen " (De Bary). 



Much of the recent work in normal and experimental embryology, 

 as well as that on regeneration, indicates that the same is true in prin- 

 ciple of animal growth. Among recent writers who have urged this 

 view should be mentioned Rauber, Hertwig, Adam Sedgwick, and 

 especially Whitman, whose fine essay on the Inadequacy of ilic Cell- 

 theory of Development ('93) marks a distinct advance in our point of 

 view. Still more recently this view has been almost demonstrated 

 through some remarkable experiments on regeneration, which show 

 that definitely formed material, in some cases even the adult tissues, 

 may be directly moulded i?ito new structures. Driesch has shown 

 ('95, 2, '99) that if gastrulas of SpJicerechinus be bisected through the 

 equator so that each half contains both ectoderm and entoderm, the 

 wounds heal, each half forming a typical gastrula, in which the ente- 

 ron differentiates itself into the three typical regions (fore, middle, 

 and hind gut) correctly proportioned, though the whole structure is 

 but half the normal size. Here, therefore, the formative process is in 

 the main independent of cell-division or increase in size. Miss Bickford 



