OUTLINES OF THK DEVEI-OPMENT OF THE TTJATARA, 47 



opens to the exterior by a narrow aperture (fig. 86, 0. P. A.) 

 a short way in front of the allantois. No proctodseum is yet 

 recognisable, and the neurenterie canal no longer opens into 

 the alimentary canal below. 



No. 47 has about thirty mesoblastic somites recognisable 

 in the stained embryo viewed as a transparent object, but they 

 are very ill-defined at the hinder end, so that it is impossible 

 to count them exactly. 



Sagittal sections through the head at this stage are very 

 instructive. It would be out of place to describe them in 

 detail here, but the following features, shown in figs. 90 and 

 91, are perhaps worthy of mention. In the fore-brain the 

 cerebral hemispheres (C. H.) and infundibulum {Inf.) are 

 clearly recognisable, the latter projecting from the thalamen- 

 cephaloii towards the pituitary body {Pit.). The hind brain 

 {H. B.) is partially divided transversely into at least six neuro- 

 meres. The notochord extends about as far forward as the 

 pituitary body, its extremity lying above and just in front of 

 the stomodseum and below the anterior part of the hind brain. 

 The pituitary body {Pit.) is just commencing its development 

 as a thickening of the front wall of the pharynx, composed of 

 columnar cells and extending forwards between the infundi- 

 bulum and the front end of the notochord. The two head- 

 cavities {H. C.) on each side are very conspicuous. The 

 anterior pair are connected across the middle line by a short 

 transverse canal (fig. 90, H. C. C.) which lies just in front of 

 the end of the notochord, while each of the posterior pair 

 gives off a conspicuous branch {H. C. M.) into the mandibular 

 arch. 



Stage M (figs. 92, 93). 



To this stage I refer two embryos, viz. No. 51, col- 

 lected on Stephens Island on November 30th, 1897, and 

 removed from the egg on January 3rd, 1898; and No. 81, 

 collected about the end of November, and removed from the 

 egg on December 27th, 1897. 



No. 51 (figs. 92, 93) may be taken as typical of the 



