VI DISPERMIC EGGS 167 



respectively) and found that these larvae frequently or generally showed 

 various and different abnormahties in one or more quarters (or thirds) of 

 their bodies. That is to say, the regions of the larval bodies derived from 

 the first four (or three) blastomeres were different from each other (Fig. 76). 



Finally, Boveri found that the quarters or thirds derived from different 

 blastomeres often had nuclei of different sizes. Now, as Boveri has shown, 

 in Echinoid larvae the surface of the nucleus is proportional to the 

 number of its chromosomes. Hence the different sizes of the nuclei in 

 various regions of the body give additional evidence that the blastomeres 

 from which they were derived differed as to the number of their chromo- 

 somes. To give specific examples : he calculated the number of chromo- 

 somes present in the nuclei of four larvae derived from whole triaster 

 embryos and found that the numbers in the three thirds derived from 

 the three primary blastomeres were as follows in the different larvae : 

 (i) 18, 36, 54 ; (2) 18, 45, 45 ; (3) 29, 43, 36 ; (4) 28, 40, 40. Fig. 76. D, 

 shows the first of these larvae. 



Now, as we have seen, cytoplasmic differences between the first four 

 or three blastomeres of tetraster and triaster eggs cannot account for 

 the varying manner in which the primary blastomeres develop. It 

 follows therefore that we must look to the nuclei for the cause of this 

 variation. A moment's consideration shows us that it is not the number of 

 the chromosomes that is of primary importance, since we have abundant 

 proof that, at any rate, n, 2n, ^n or ^n chromosomes are perfectly 

 compatible with a normal organism (see Chapter V., etc.). It is only very 

 rarely that even one, and still more rarely that two, of the primary 

 blastomeres of a tetraster or of a triaster will have less than n chromo- 

 somes. Moreover, the four triaster larvae, the number of whose chromo- 

 somes are given in the preceding paragraph, were normal in all their 

 parts, although the number of chromosomes was so varied. We are 

 left with the conclusion that the cause of the abnormalities is the luck 

 of certain members of the chromosome series in certain of the primary 

 blastomere nuclei (cf. Fig. 75, E). The fact that the abnormalities which 

 arise in development are not identical in all the blastomeres of a single 

 egg, is due to the fact that different members of the series arc lacking 

 in the different blastomeres. This, then, is direct experimental evidence 

 of the functional differentiation of the chromosomes. 



C. THE PROCESS OF MEIOSIS 



If we take it that the hereditary substance is composed of smaller 

 differentiated elements, each with its own particular function, it is 

 plain that some arrangement must be sought by which a doubhng of 

 the number of these elements in each successive act of syngamy is 



