154 THE LATER STAGES OF DEVELOPMENT 



distinguishable (Stohr, 1924). Explanted parts of cephalopod 

 (cuttle-fish) embryos also proved able to continue their differ- 

 entiation in the normal way (Ranzi, 1931). 



A somewhat different situation obtains in tissue cultures. 

 Here the tissues are kept in a medium specially provided with 

 food and growth promoting substances. This stimulation of 

 growth inhibits differentiation. Therefore, embryonic tissues 

 are usually unable to differentiate in these cultures, and such 

 structure as may already be present in the tissues will even be 

 reduced so that the cells become less differentiated in their 

 external appearance. However, under certain circumstances, 

 differentiation is possible in tissue cultures. Gaillard (1931), 

 for instance, has shown that the differentiation of bone-forming 

 tissue is promoted by the successive addition to the culture of 

 a series of extracts from embryos of steadily increasing age. 



In this context, it is important to note that the loss of visible 

 cell differentiation in tissue cultures does not imply that the 

 cells really relapse into an undifferentiated, embryonic con- 

 dition. It is true that cells determined into different directions 

 e.g. bone-forming cells, and cells of the heart-primordium, in 

 tissue cultures become indistinguishable in external appearance, 

 but even after a very prolonged stay, they still retain their 

 different properties, and as soon as circumstances permit they 

 return to their particular course of differentiation. There is no 

 real "de-differentiation'\ therefore, but only ''modulation" (P. 

 Weiss, 1950). 



We have already seen that the beginning of the mosaic stage 

 is soon followed by visible tissue differentiation. After this, 

 growth becomes gradually more important. Admittedly the 

 embryo may have increased in size before this stage, but this 

 was mainly due to water uptake, and was more a swelling 

 process, than real growth. True growth cannot begin until 

 differentiation has proceeded so far that the embryo is able to 

 take up food independently, or at least, until its blood circula- 

 tion is well enough developed for the transport of food from 

 the yolk, or from the maternal tissues, toward the embryo to 

 be possible. In other words, the period of differentiation 

 generally precedes growth. 



