200 ESSENTIALS OF BIOLOGY 



separate cells. The addition of a little lithium salt to the water 

 in which the same embryos develop may cause the archentron 

 to be evaginated instead of invaginated. 



The ovum may be fertilized by more than one spermatozoon 

 — ^when notable abnormal embryos are developed. Doubtless the 

 '' heredity " of ovum and sperm may also lead to abnormalities 

 — thus the occurrence of extra digits, etc., in a family. We 

 consider such cases in later chapters. On the whole, just as with 

 the general problem of development, there is no adequate theory 

 of abnormalities, or disharmonies. 



72fl. Regulations of Development. Yet interference with 

 the normal course of an embryogeny may not affect the formation 

 of a normal larva. Some examples of such drastic interferences 

 will be given and it will be seen that the embryo has a certain 

 degree of power to compensate for these events and to affect 

 regulations of embryogeny that lead to the normal result. There 

 are limits to this power of regulation and these also we shall 

 notice. 



(i) The sea-urchin egg and embr\^o possesses power of regula- 

 tion in a marked degree. A normal segmentation of the ovum 

 results in an 8-cell embryo. At this stage the embryo is a " har- 

 monious equi-potential system." If development be followed 

 out it will be found that the eight blastomeres have certain fates, 

 in that they are the precursors of different parts of the larva that 

 normally forms : the various blastomeres develop in a certain, 

 specific harmony. Nevertheless, these eight cells can be shaken 

 apart from each other and then a regulation is effected. Each 

 of them begins anew the process of embryogeny so that eight 

 perfect but dwarf gastrula-larvae come into existence. Therefore 

 although each of the original cells of the segmented ovum has a 

 " prospective value," which becomes realized in the normal 

 development, it is also equipotential with all the other cells in 

 that, if it should be necessary, it has also their prospective values. 



(2) In the normal 8-cell stage (Fig. 26) the blastomeres have 

 certain definite positions relative to each other. In the normal 

 development what a blastomere is going to become depends on 

 its position relative to the others. Thus in some embryos the 

 cells on one side of a certain plane become the right-hand half 

 of the body and vice versa. If now, the embryo at, say, the 

 4-cell stage be lightly compressed between two plates of glass 



