1 142 REPRODUCTION 



(Heape), and in women some development of the mammary glands 

 is often associated with the menstrual period. The stimulus to the 

 development of the gland in these cases appears to be some change 

 correlated with oestrus, and cannot be a change correlated with 

 pregnancy. 



Cultivation of Tissues outside of the Body.- Closely related to the 

 marvellous power of growth of the fertilized ovum in the favourable 

 nidus of the pregnant uterus, although, of course, incomparably 

 inferior, is the power of growth and reproduction of isolated tissue 

 cells in a suitable medium outside of the body. An instance of this 

 has already been described in the case of nerve-cells (pp. 803, 857). 

 Many other tissues have been successfully cultivated in sterile 

 coagulated lymph or blood-plasma. Connective-tissue cells grow 

 very easily, and can apparently be preserved indefinitely in the 

 living state. A strain of these cells, originally obtained from a 

 fragment of the heart of an embryo chick which had been pulsating 

 in vitro for 104 days, has been seen to proliferate rapidly outside of 

 the organism for more than sixteen months, and after more than 

 190 passages into fresh media. At the end of this time the rate of 

 proliferation of the connective-tissue cells was even greater than 

 that of fresh connective tissue taken from an embryo eight days old. 

 Extracts of tissues and tissue juices under certain conditions acceler- 

 ate the growth of connective tissue from three to forty times, the 

 growth being measured by the increase in area of the minute pieces 

 of tissue. This activating power is especially marked in extracts 

 of embryos, of adult spleen, and of certain sarcomas. This is 

 noteworthy as the great characteristic of malignant tumours is 

 their indefinite power of growth. The activating substance is 

 unable to pass through a Chamberland filter (Carrel). Cultures of 

 adult tissue have a smaller power of persistent growth. In the 

 majority of cases growth in plasma without the addition of a 

 stimulating tissue extract ceases after three or four generations 

 (Walton). 



The time of survival of tissues at low temperatures under con- 

 ditions which do not encourage growth is also a matter of consider- 

 able interest, both from the physiological and the practical point of 

 view, since living sterile tissue is required for a number of surgical 

 operations for example, skin for grafting. Skin has been suc- 

 cessfully grafted after being kept two to seven weeks in cold storage, 

 but after a longer period there were many failures. Embryonic 

 chick and rat tissues live longest at about 6 C., but not more than 

 twenty days under the most favourable conditions, according to 

 Lambert. 



Transplantation of Tissues. Besides the growth and regeneration 

 of tissues or organs, the simple displacement of them from their 

 normal situation and their implantation in a new environment have 



