4 ELEMENTS OF HISTOLOGY. [Chap, i 



(Figs. 2, SA, SB), so that after a few days we find 

 within the original investment of the ovum a large 

 number of minute elements, each consisting of 

 protoplasm, and each containing a nucleus. 



3. From these elements, which become smaller as 

 the process of cleavage progresses, all parts and organs 

 of the embryo and its membranes are formed. It can 

 be easily shown that the individual elements possess 

 the power of contractility. Either spontaneously or 

 under the influence of moderate heat, electricity, 

 mechanical or chemical stimulation, they throw out 

 processes and withdraw them again, their substance 

 flowing slowly but perceptibly along. Hence they 

 can change their position. In this respect they com- 

 pletely resemble those lowest organisms which are 

 known as amoebae, each of these being likewise a 

 nucleated mass of protoplasm. Wherefore this move- 

 ment is termed amoeboid movement. It can be further 

 shown that they, like amoebae, grow in size and divide 

 that is to say, the individuals of a generation grow 

 in size before each gives rise to two new daughter 

 individuals. 



4. Although for some time during embryonic life 

 the elements constituting the organs of the embryo are 

 possessed of these characters, a time arrives when 

 only a limited number of them retain the power of 

 contractility in any marked degree. At birth only the 

 white corpuscles of the blood and lymph, many of the 

 elements of the lymphatic organs, and the muscular 

 tissues, possess this power, while the others lose it, 

 or at any rate do not show it except when dividing 

 into two new elements. Some of these elements 

 retain their protoplasmic basis ; as a rule, each con- 

 tains one nucleus (but some two or more) and is 

 capable of giving origin by division to a new genera- 

 tion. Others, however, change their nature altogether, 

 their protoplasm and nucleus disappear, and they 



