108 SUMMARY OF QUERENT RESEARCHES RELATING TO 



Growth in Length.* — The late Richard Assheton delivered three 

 lectures in 1913 on the growth in length of the Yertebrate embryo, 

 which have now been published, along with a reprint of a paper on the 

 mechanics of gastrulation. Anatomical observation and experiment 

 point to the conclusion that. in all anamniote chordates the growth in 

 length of the embryo is due to the origin of a special area of cell- 

 production rouud the lips of the blastopore which converts the spherical 

 form of the gastrula into the cylindrical form of the later embryo. 

 Since this area comes into being only after the gastrula is formed, .we 

 may recognize two regions in the later embryo. One of these, the 

 protogenetic region, is the direct result of the segmentation of the ovum 

 culminating in the gastrula, and having the general character of a 

 radially symmetrical form, and this on the whole is to be identified 

 with the coelenterate phase of evolution. The other region, called 

 deuterogenetic, is produced by proliferation of the lips of the gastrula 

 mouth. 



The part formed from the protogenetic region includes the fore- 

 brain, probably also the mid-brain, the mouth, and possibly the hind- 

 brain as far as the origins of the fifth and eighth nerves, the branchial 

 region and heart, and probably much of the gut. The part formed 

 from the deuterogenetic region comprises the remainder of the hind- 

 brain, the spinal cord, the tail, the whole of the metamerically segmented 

 mesoderm, and in the crcmiates the renal organs. There is much evidence 

 to show that in the craniate chordates the actual germ-cells are pro- 

 togenetic in origin, but migrate during development into the deutero- 

 genetic region and here undergo their maturation, eventually finding 

 their way to the exterior by means of the deuterogenetic channels of 

 the cojlom or renal apparatus. 



The same relations between the two regions probably hold good for 

 the amniotes, though in them experimental evidence is obtained less 

 easily. 



Having attained the stage of a ccelenterate, the gastrula may take 

 diverse courses. The whole deuterogenetic centre might continue active, 

 or it might cease to be active almost as soon as formed, or a portion 

 might remain for a longer time active while the rest dies out earlier. 



It may be suggested that deuterogenesis in the embryo is a geo- 

 metrical consequence following gastrulation (which may perhaps also be 

 regarded as a necessary culmination of protogenesis). If so, these con- 

 sequences of embryonic development must have had their effect upon 

 evolution. For instance, one of the first effects of deuterogenesis must 

 be in many cases a tendency to close the blastopore. 



Now, clearly, in evolution, to close the only entrance to the gut 

 would be fatal. Therefore the necessity to live must have opposed the 

 geometrical tendency. The warring of the two tendencies may well 

 have given the impetus in evolution which has led to the very varying 

 fates of the blastopore in the different groups, and the consequent rela- 

 tion of the main axis to the plane of the blastopore. 



The two phenomena, the difference in the fate of the blastopore, 



* Growth in Length : Embryological Essays. Cambridge: (1916) xi and 104 pp.. 

 (42 figs.). 



