488 MISS K. M. PARKER ON THE 



backward concurrently with the forward growth of the brain- 

 plate. He gives a series of comparative measurements which 

 show that the rate of removal of the lip of the anterior intestinal 

 portal from the anterior end of the brain is greater than the rate 

 at which the brain-plate grows forward from a given fixed point ; 

 hence it is evident that the anterior intestinal portal must be 

 moving backwards. 



Concurrently with the formation of the fore-gut, the lateral 

 heart-tubes come to lie ventrally to it, but do not at once fuse. 

 Wilson (20), in a paper on young human embryos, draws attention 

 to this fact and refers to the embryo of Perameles iiastcta, described 

 in this paper as Stage III., as exemplifying this condition ; but 

 he does not discuss the question as to how these lateral hearts 

 approach one another. 



We may now consider the evidence afforded by the material 

 described above, and will endeavour to show that it is entirely 

 in accord with the view that there is actual backward growth of 

 the anterior intestinal portal, and that it is this process, and 

 not fusion of lateral folds, that brings about lengthening of the 

 fore-gut. 



It may be noted here that in Perameles, as in the rabbit 

 (Rouviere), no ventral mesocardium is present at any stage, a fact 

 which, in itself, is a strong argument against the theory that gut- 

 closure is effected by the fusion of lateral folds. 



If we compare a stage in which the head-fold has not yet 

 appeared (PI. I. figs. 1 & 2) with one in which a small portion 

 of the fore-gut is differentiated (figs. 3 & 4), we see that the pleuro- 

 pericardial ccelom not only moves backward relatively to the 

 brain-plate but also increases very considerably in width. It is 

 obvious that such an increase in size must either cause the 

 pericardium to extend peripherally or to close in towards the 

 axial line, and it is perfectly clear on comparison of figs. 2 & 3 

 that it is this latter process which is taking place. From a 

 longitudinal section, such as is shown in text-fig. 4, it is evident, 

 moreover, that such an expansion of the pericardium must involve 

 the backward growth of the splanchnopleural floor of the fore- 

 gut. If the lengthening of the fore-gut were due entirely to the 

 rapid forward growth of the brain-plate, there would be no such 

 inward closure of the pericardial region. Moreover, if we compare 

 fio-s. 2 & 3 (PI. I.), we see that in the earlier stage, the pericardial 

 coelom is situated peripherally to the margin of the brain-plate, 

 while in the second stage, the inner margin of the pericardium 

 lies in the lip of the anterior intestinal portal. Now the growth 

 in length of the brain-plate in the period between these two 

 stages would naturally give rise to a fold round its anterior 

 margin, but would not bring the pericardium into the position it 

 occupies in Stage II. (fig. 3), unless there occurred concurrently 

 with such growth in length either an increase in width of the 

 brain or an inward closure of the pericardium. Comparison 

 of figs. 2 & 3 again shows that while no increase in width of the 



