490 MISS K. M. PARKER ON THE 



the brain-plate initiates the formation of the head -fold, there 

 occurs concurrently with this process in the mammal, and at a 

 slightly later stage in the chick, a rapid expansion of the peri- 

 cardium and a consequent backward and inward growth of the 

 fold of splanchnopleure which constitutes the inner margin of 

 the pleuro-pericardial cavity. 



Additional evidence is afforded by the study of the longitudinal 

 sections and reconstructions of Stages III. & IV. (text-figs. 12 cfe 

 15 ; PI. I. figs. 4 & 5). From the longitudinal sections, it is evident 

 that a great increase in length of the brain has occurred in the 

 mid- and fore-brain regions. If, therefore, the increase in length 

 of the fore-gut were due to elongation of the brain-plate, a corre- 

 sponding increase should occur in the portion of the fore-gut 

 lying below these segments, i. e. the portion anterior to the first 

 visceral pouch in Stage III. (fig. 4). Comparative measurements 

 of the gut in figs. 4 & 5 show, however, that no increase in length 

 has occurred anterior to the first visceral pouch. Moreover,, 

 growth of the medullary plate would not necessarily bring about 

 lengthening of the fore-gut unless it occurred along a straight line 

 representing the longitudinal axis of Stage III., i.e. unless the 

 brain remained unflexed. The positions of the auditory neuro- 

 mere in fig. 4 (opposite the second visceral pouch) and fig. 5 

 (opposite the first visceral pouch), show that the brain-plate 

 has moved forward relatively to the gut between Stages III. & 

 lY. If, now, we study the longitudinal section of Stage lY. 

 (text-fig. 15) we see the conditions resulting from the increase in 

 length and forward growth of the brain-plate. The gut has 

 increased in dorsi-ventral extent, the medullary plate projects 

 considerably anterior to the cephalic limit of the gut, and 

 flexure has occurred at two points ; that is to say, the rapid 

 growth of the fore- and mid-brains, instead of involving a longi- 

 tudinal stretching of the portion of the gut lying ventral to 

 them, has caused little or no increase in length of the embryo 

 along its straight long axis : the additional extent of the brain- 

 plate is accommodated within the limited space by flexure. 



We see, therefore, that although the brain-plate lengthens 

 rapidly after the first establishment of the head-fold, we can safely 

 conclude that this does not cause elongation of the gut, for the 

 regions of greatest growth of the gut and brain-plate are not 

 correlated and the A^alue of the forward growth of the brain as a 

 factor in the lengthening of the fore-gut is largely rendered 

 niigatory by the occurrence at this period of the cranial flexure. 



On the other hand, there is little difiiculty in interpreting the 

 progressive closure of the gut at this stage as being due to an 

 entirely different cause, for the median pericardium has extended 

 rapidly, its antero-posterior length in the middle line having 

 more than doubled in the short developmental period elapsing 

 between Stages III. and lY. (PI. I. figs. 4 & 5). As the anterior 

 margin of the peiicardium is in contact with the ectoderm of the 

 head-fold, the rapid expansion of the cavity naturally involves a 



