352 PRINCIPLES OF EMBRYOLOGY 



has on nuclei. There are a few well-known cases in which the reactions 

 produce alterations of the nucleus which are easily visible in conventional 

 microscopical preparations (Review: Mather 1948a). One of the best 

 known of these occurs in the eggs of Ascaris, a nematode parasite in the 

 gut of the horse. This possesses only one pair of large chromosomes 

 (polyploid races with two or four pairs are also knovm). These chromo- 

 somes are not very typical ones; in place of the normal single centromere 

 or 'spindle fibre attachment' they are provided with a whole series of 

 them extending along the centre section of each long chromosome. A 

 visible nuclear differentiation takes place very early, since from the two- 

 cell stage onwards the chromosomes in most cells break up into small 

 fragments, each provided with only one centromere, and the distal centro- 

 mere-less ends (which are heterochromatic) are thrown out of the nucleus 

 into the cytoplasm. Only in that lineage of cells which eventually gives 

 rise to the germ cells do the long chromosomes retain thek original con- 

 figuration (Fig. 16.3). Some earlier authors (e.g. Zur Strassen) suggested 

 that the phenomenon was due to factors residing in the chromosomes 

 themselves, supposing that at each cleavage mitosis there was an unequal 

 and orientated division by which a specially coherent chromosome was 

 segregated into this lineage of cells. But Boveri, who was the first to des- 

 cribe this process of 'chromosome diminution', showed by a study of 

 abnormal cleavages in dispermic and centrifuged eggs that the retention 

 of the original structure is dependent on the type of cytoplasm into which 

 the nucleus moves (discussion in Schleip 1929). The type of cytoplasm in 

 which the chromosomes remain coherent can, it is claimed, be recognised 

 not only by its location in the egg but by the presence of fewer vacuoles 

 than there are in the rest of the egg (Bonoure 1939). 



Very similar examples of a rapid differentiation of nuclei following a 

 single division can be seen in a few other animals and in the pollen grain 

 formation of some plants. 



In certain animals, differences can be seen between the nuclei in different 

 tissues. It is indeed usual for the nuclei to differ somewhat in size, general 

 intensity of staining and perhaps in the number or size of nucleoli, but 

 the significance of these characters is obscure. A more defmite type of 

 difference, found in certain groups, is the formation of polyploid nuclei, 

 containing multiples of the normal diploid number of chromosomes. 

 This is rather common in insects (Reviews: White 1954, Geitler 1948). 

 In many tissues the chromosomes divide, and something rather like a 

 process of mitosis may occur in an abbreviated form without any accom- 

 panying division of the cell body or of the nucleus. The resulting chromo- 

 some threads may lie in a loosely packed mass, or each may adhere closely 



