Groivth of Mammalian Tissue in Vitro. 



481 



With the liver there were positive results in 9 per cent, of the cases, but 

 even in these growth was extremely slight. In no case were any cuboidal 

 or parenchymatous cells seen. There was very slight growth of connective 

 tissue, so that after three days there could be seen in a few cases several long 

 connective tissue cells growing here and there from the edge of the tissue 

 (fig. 4). Often a portion of the original tissue stained poorly and was mani 

 festly dead. 



These experiments, therefore, strongly confirmed the observations of 

 previous workers, namely, that the tissue of young or embryonic animals 

 shows more active growth than similar tissues taken from adult animals. 

 In addition to this, it appeared manifest that the plasma of such young 

 animals was not nearly so suitable a medium as the plasma of adult animals. 

 Such a condition has not previously been described, and is so opposed to what 

 one would at first sight believe, that it seemed necessary to confirm these 

 experiments. For this purpose further experiments were carried out with 

 stock cultures of rabbit testicle. These tissues had been growing in vitro for 

 10 generations in a medium composed of two parts of plasma and one part 

 of spleen extract. At this period they were growing vigorously, so that at 

 the end of 48 hours there was a wide zone of newly formed cells surrounding 

 the original tissue. 



Experiment 13. — Stock specimens of rabbit testicles as described above were 

 cultivated in groups of four each — (a) in plasma obtained from a young 

 animal 10 days old ; (b) in plasma obtained from an adult rabbit over a 

 year old ; (c) in a mixture of plasma from an old animal, two parts, and 

 spleen extract one part. At the end of 48 hours, the specimens cultivated in 

 the mixture of old plasma and spleen extract were growing vigorously and 

 showed a very wide zone of active cells, mainly of the connective tissue type 

 (see fig. 5). These specimens served as a control. Those growing in old 

 plasma also showed a wide zone of cells of the connective tissue type, but 

 the newly formed cells, as was to be expected, owing to the absence of a 

 stimulating extract, were considerably fewer in number and did not extend 

 so far into the surrounding media (see fig. 6). 



In the case of the tissues growing in young plasma, growth was very 

 slight. There were only a few cells growing from the edge of the tissue (see 

 fig. 7) and in these cells but few mitotic figures were seen. 



This experiment confirmed those carried out with fresh tissues, and there 

 can be no doubt that young plasma is a less suitable medium for the growth 

 of tissues than that of old animals. It has been previously shown* that 

 with older animals some plasmata are not such good media as others, and 

 * ' Roy. Soc. Proc.,' B, vol. 87, p. 452 (1914). 



