443 LECTURE XVIII. 



delicate a nature, that we are not as yet able to demon- 

 strate them. 



In the development of the embryo a phenomenon has 

 been known for years which positively indicates the ex- 

 istence of such differences in the formative cells, inas- 

 much as the different segments of the ovum run through 

 their phases of development with different degrees of 

 rapidity, and especially those parts which are destined 

 to form the higher organs, run through their individual 

 stages with much greater celerity than those whose lot it 

 is to form the lower tissues. In the size of the cells 

 also differences seem to exist. In a similar manner we 

 frequently see that in pathological formations also differ- 

 ences occur in reference to the time occupied in their de- 

 velopment. Whenever the development of the cells 

 takes place with great rapidity, we may be sure it is a 

 more or less heterologous development. An homolo- 

 gous, hyperplastic formation always presupposes a cer- 

 tain tardiness in the processes which give rise to it ; 

 the cells generally remain of a larger size, since the di- 

 visions do not usually proceed until very small forms are 

 produced. 



Though so extremely simple in nature and in theory, 

 these modes of development are certainly extremely dif- 

 ficult of demonstration in individual places. The parts 

 which apparently ought to be the most conveniently 

 situated for the purpose of investigation, and in which 

 Henle indeed, as long as twenty years ago, all but made 

 the discovery of such a development, are the epithelia. 

 Here, where a development often so abundant takes 

 place upon the surface of a membrane, one would sup- 

 pose it must be extremely easy to trace its course accu- 

 rately in the individual cells. Henle, you know, en- 

 deavoured to show that mucus-corpuscles, and indeed 

 many forms which belong to pus, were produced on the 



