The philosophy of biology . c5rtoplasm. Within the nucleus isa substance distinguishablefrom the rest; it is distri-buted in granules and is calledthe chromatin (i). When thecell is about to divide thischromatin becomes arrangedin a long coiled thread (2), andthen (3) this chromatic threadbreaks into short rods called chromosomes. Two littlegranules now appear, one at each end of the nucleus, andvery delicate threads, the asters, appear to pass from eachof these bodies towards the chromosomes (4). Eachof the latter then splits lengthways into two, and ahalf chromosome appears to be drawn by th
The philosophy of biology . c5rtoplasm. Within the nucleus isa substance distinguishablefrom the rest; it is distri-buted in granules and is calledthe chromatin (i). When thecell is about to divide thischromatin becomes arrangedin a long coiled thread (2), andthen (3) this chromatic threadbreaks into short rods called chromosomes. Two littlegranules now appear, one at each end of the nucleus, andvery delicate threads, the asters, appear to pass from eachof these bodies towards the chromosomes (4). Eachof the latter then splits lengthways into two, and ahalf chromosome appears to be drawn by the asterstowards the poles of the nucleus. The latter thendivides (5) and then the whole cell divides. Whatthus, in essence, happens in nuclear divisions is thatthe chromatin of the nucleus is more or less accuratelyhalved. Apparently this substance consists of veryminute granules and the whole process is directedtowards the splitting of each of these granules into half-granule then goes to each of the daughter Fig. 13. THE VITAL IMPETUS 131 Every time the embryo divides this process is each of the (theoretically) 1028 cells of theblastula contains iw^th of the substance of eachchromatic granule in the fertilised ovum. Pfluger and Roux (in 1883 and 1888 respectively)were the pioneers in the experimental study of thedevelopment of the ovum, and the results of theirwork and that of their successors has, more than any-thing else in biology, modified and shaped our notionsof the activities of the organism. Roux found, orthought so at least, that the first division of the frogsegg marked out the right and left halves of the body,the one blastomere giving rise to the right half, theother to the left half. The next division, whichseparates each of these blastomeres, marked out theanterior and posterior parts of the embryo. Thus :— LeFf y PN^ ^^^.^Anteriorf--- ^^Anfenor
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