THE DEVEI-OPMENT OF S YM HRANOHUS MARMORATU8. 21 
its internal limits, but tlie balance of evidence from many 
sections seems to show that it does not extend inwards 
beyond the infundibulum and the hypophysis. The outer- 
most horny layer is not involved in the growth in any way. 
The stomodmum is, of course, quite solid. The flat, laterally 
spreading endoderm of the preceding stages is becoming 
a more or less rounded, solid mass of cells in the stomach 
rudiment, which is very closely apposed to the overlying 
notochord. 
The pharyngeal rudiment overlies the pericardium in Stage 
19. The solid endodermal cleft rudiments are not yet clearly 
distinguishable. 
Transverse sections of embryos of Stage 21 show that the 
anterior parts of the alimentary canal, including the visceral 
clefts, are gradually moulded during the course of develop- 
ment out of a solid mass of tissue, swollen out laterally and 
tapering towards the central axis, which begins posterior to 
the infundibulum. The position of the solid hyobranchial cleft 
rudiment can now (Stage 21) be defined, while behind the 
branchial arches the rudiment of the oesophagus can be dis- 
tinguished as a solid mass, broader than deep. Posteriorly, a 
solid cord of endoderm cells, the gut-rudiment, stretches with 
little change, save a gradually decreasing diameter to the 
hind end. The process of separation of this definitive endo- 
derm from the syncytial yolk-endoderm has not been 
observed. 
Idle alimentary canal is still solid at Stage 24, the cloacal 
opening being established by Stage 25. The lining of the 
stomodgeum at Stage 24 is differentiating into a very regular 
epithelium. Mandibular and hyoid arches can clearly be dis- 
tinguished in sagittal sections, while in the branchial rudiment, 
although the clefts have not yet been hollowed out, it is easy 
to see the number of arches, because of the epithelial lining 
of each. 
Transverse sections at Stage 24 are particularly inter- 
esting because they show clearly an ingrowth of ectoderm in 
connection with each of the branchial cleft rudiments. The 
