150 THE GERM -PLASM 



a very small share in the reconstruction of the anterior and 

 posterior ends of the animal during the process of fission, the 

 restoration being effected in this case chiefly by the mesoderm, 

 or so-called ' connective tissue ' cells, which ' are suspended in 

 large numbers in tlie perivisceral fluid between the supporting 

 trabeculae.' These cells begin to increase in number when the 

 animal is preparing for division, and by their multiplication 

 they form a ventral mass of so-called ' embryonic ' cells, which 

 gives rise to the pharynx, the pharyngeal and prostomial glands, 

 all the parts generally known as ' parenchymatous ' or ' meso- 

 dermic structures,' and also apparently to certain parts of the 

 nervous system. Kennel * found that a similar mode of develop- 

 ment of these parts occurs in a Planarian. In such cases we 

 must therefore suppose that the accessory determinants required 

 for regeneration are supplied to the mesoderm cells, instead of 

 to those of the ectoderm. We cannot at present determine 

 whether this is effected by each of these cells being provided 

 with all the supplementary determinants for the mesoderm, 

 which only become disintegrated and distributed amongst the 

 other cells when these begin to multiply, or — as we assumed in 

 the case of the ectoderm cells of A^ais — by the distribution of 

 the different determinants to a number of these cells before pro- 

 liferation occurs. 



The regularity with which all organs are formed in the proper 

 position and mutual relation, may perhaps be taken as a proof 

 of the assumption that they contain latent determinants which 

 are from the first separate, and which differ according to the 

 topographical position of the organ. It is hardly possible that 

 the contrary assumption can be the correct one, for this would 

 render it necessary to suppose that although all the determinants 

 are certainly present in every formative cell, only that one can 

 undergo development which corresponds to the region in which 

 the cell happened to be situated. 



Here, again, we meet with no serious difficulty as regards 

 the derivation of the required supplementary determinants in 

 ontogeny : in fact there is less difficulty in this case than in that 

 oi Nais, for the cells of the different layers of the body contain 

 the determinants for the correspondijig organs. 



* J. Kennel, ' Untersuchungen an neuen Turbellarien,' Zool. Jahrbiicher, 

 Bd. 3. Abth. f. Anat. u. Ontog. d. Thiere, p. 447. 



