SEGMENTATION OF THE OVUM 45 



adult state by means of a longer or shorter series of cell-divisions. 

 The earlier cell-divisions constitute what is termed the segmenta- 

 tion of the ovum, and result in the formation of a number of 

 daughter cells, which exhibit little or no differentiation amongst 

 themselves and are known as blastomeres. Sooner or later, 

 however, differentiation sets in and leads to the formation of 

 more or less highly specialized tissues or cell aggregates. 



In the primitive, fish-like Amphioxus, 1 for example, the ovum 

 (Fig. 13, I) is a spherical, nucleated cell about 215 o^h inch in 

 diameter. After fertilization by a male gamete or spermatozoon 

 it divides first into two equal and similar embryonic cells or 

 blastomeres (Fig. 13, II) by a vertical cleavage. Another vertical 

 cleavage, at right angles to the first, divides each blastomere 

 into two smaller ones (Fig. 13, III). This is followed by a hori- 

 zontal cleavage, which results in the formation of eight cells, 

 in two tiers of four each, the four upper ones being slightly 

 smaller than the four lower (Fig. 13, IV). The blastomeres go on 

 dividing and presently arrange themselves in the form of a 

 hollow sphere, whose wall is composed of a single layer of cells 

 (Fig. 13, VII). The embryo has now reached the blastula (or 

 blastosphere) stage of its development, a stage which is passed 

 through by all multicellular animals whose life-history follows 

 a typical course unmodified by secondary features. 



If we allow for the fact that the cells all remain together 

 instead of separating from one another after each division, it is 

 obvious that the segmentation of the fertilized ovum into blasto- 

 meres is identical with the process of multiplication by fission in 

 such a protozoon as Amo3ba. The fact that each blastomere is 

 equivalent to a single protozoon and multiplies in a similar 

 manner has been experimentally demonstrated in an extremely 

 interesting way by Herbst, whose observations were made upon 

 the development of the common sea urchin (Echinus). He 

 found that, if the eggs are allowed to develop in sea water from 

 which every trace of calcium has been removed, the blastomeres 

 actually do separate after each division and give rise to 808 

 individual cells, which swim about separately like so many 

 flagellate Protozoa instead of remaining united together and 

 co-operating with one another to form the normal blastula. The 

 blastula stage itself, of course, corresponds very closely in general 

 features with such a protozoon colony as we meet with in the case 



1 The external appearance of the adult Amphioxus is represented in Fig. 118. 



