VI HYBRIDS 187 



who also examined the spermatogenesis of the horse. That of the ass 

 has, however, not yet been described. The number of chromosomes 

 (diploid) in the mule is 51, one being a large unpaired X chron: 

 As 2n in the horse is 36 + X, the ass may be presumed to possess O4 r \ 

 chromosomes. Evidences of physiological disturbance appear early in 

 the mciotic prophase of the mule, the diplotcnc stage found in the horse 

 being replaced by a reticular stage showing only occasional and irregular 

 duplicity of threads. The majority of cells perish in this stage ; in 

 those that reach the late prophase the number of chromosomes varies 

 between 34 and 49, the commonest numbers being 40 to 45. i.e. there are 

 about 5 to 10 bivalents, the remainder being univalents. The few cells 

 that reach metaphase I., or even anaphase I., are marked by numerous 

 abnormalities, especially multipolar mitoses and the failure of many of 

 the chromosomes to gain attachment to the spindle fibres. No secondary 

 spermatocytes or any later stages were ever observed. 



F. THE NUCLEUS IN MORPHOGENESIS 



All arguments in favour of the nucleus being the bearer of the 

 hereditary factors are of course equally arguments for its control of 

 morphogenesis. As to its mode of action in this respect, however, we 

 are almost wholly ignorant. One thing which does seem certain is that 

 all the nuclei of the body — at any rate up to a late stage of development 

 — are identical in their potentiaHties, i.e. contain a complete (double) 

 set of hereditary factors. There are no differential nuclear divisions 

 in the embryo by which the endoderm factors are sorted out into the 

 nuclei of the cells which are to form endoderm, mesoderm factors into the 

 mesoderm nuclei, etc., as originally supposed by \\'eismann and Roux. 



This has been abundantly proved by the pressure experiments of 

 Driesch (1893), etc., as well as by many other well-estabhshed facts of 

 embryogeny, regeneration, etc. The particular experiments referred to 

 consisted in making sea-urchin [Echinus) eggs undergo their early 

 development under pressure between two glass plates. Under such 

 conditions the eggs may continue to develop as far as the 4th cleavage 

 (16 cells). Instead of forming a spherical blastula. however, the cells are 

 all arranged in one plane in a flat plate. On removing the pressure, 

 the embryo gradually recovers its proper shape and proceeds to normal 

 development, in spite of the fact that, as can be easily verified, the cells 

 have a quite different mutual arrangement and contain different parts 

 of the cytoplasm from those which they have in the normal larva. Thus 

 a cell which in the normal larva would have given rise to ectoderm, now 

 gives rise to endoderm, etc. 



What is it then that causes the cell differentiation which leads to the 



