84 THE BIOLOGY OF AN ANIMAL. 



makes its appearance near the nucleus, known as the attraction-sphere or 

 archoplasm-mass, and in its interior there is often a smaller body, the 

 centrosome. The first step in cell-division is the fission of the archoplasm- 

 mass into two, each containing a centrosome (derived by fission of the 

 original centrosome); after this the two masses move apart to opposite 

 poles of the nucleus (Fig. 37, B). The reticulum now becomes, in most 

 cases, resolved into a thread coiled into a skein (not shown in the figure), 

 which finally breaks up into a number of bodies known as chromosomes. 

 Their form (granular, rodlike, loop-shaped) and number (two, eight, twelve, 

 sixteen, etc., or often much higher numbers) appear to be constant for 

 each species of plant and animal. The second principal step is the longi- 

 tudinal splitting of each chromosome into halves (Fig. 37, B) and the 

 disappearance of the nuclear membrane. 



In the third place starlike rays (aster) appear in the protoplasm around 

 the archoplasm-masses, a spindle-shaped structure appears between them 

 (Fig. 37, (?), and the double chromosomes arrange themselves around the 

 equator of the spindle. The structure thus formed is known as the amphi- 

 aster or karyokinetic figure. 



Fourthly, the two halves of each chromosome move apart towards the 

 respective poles of the spindle and the entire cell-body then divides in a 

 plane passing through the equator of the spindle. Each group of daughter- 

 chromosomes now gives rise to a reticulum, which becomes surrounded with 

 a membrane and forms the nucleus of the daughter-cell. The spindle dis- 

 appears, and in some cases the archoplasm-mass, with its star- rays (aster), 

 seems to disappear also. In other cases, however, the archoplasm-mass and 

 centrosome persist and may be found in the resting cell (e.g., in leucocytes 

 and connective-tissue cells), lying near the nucleus in the cytoplasm. 



It appears from the foregoing description that each daughter-cell re- 

 ceives exactly half the substance of the mother-nucleus (chromatin), mother- 

 archoplasm, and mother-centrosome. In many cases the cytoplasm also 

 divides equally, in other cases unequally. 



It has been proved in a considerable number of cases that in the fer- 

 tilization of the ovum each germ-cell contributes the same number of chro- 

 mosomes, and the wonderful fact has been established with high probability 

 that the paternal and maternal chromatic substances are equally distributed 

 to the two cells found at the first segmentation of the ovum. It is further 

 probable that this equal distribution continues in all the later divisions ; 

 and if this is true, every cell in the whole adult body contains material 

 directly derived from both parents, and hence may inherit from both. 



Gastrulation. Germ-layers. Differentiation. Origin of the 

 Body. Almost from the first the cells arrange themselves so as 

 to surround a central cavity known as the se g mentation- camty . 

 This cavity increases in size in later stages, so that the embryo 

 finally appears as a hollow sphere surrounded by a wall consist- 



