52 
ON A PLATYPUS EMBRYO. 
the medullary plate, it gradually becomes deeper posteriorly, 
attaining its maximum depth just anterior to the 1st pair of 
somites, where it is distinctly Y-shaped (fig. 7, d. fr.). Behind 
this point it gradually becomes shallower again until in the region 
of the 8th somite the medullary plate is almost flat, and much 
thinner than anteriorly (figs. 9 and 10, md. p.). Posterior to the 
somites the plate gradually increases in breadth to form the sinus 
rhomboidalis which invests the primitive streak. The extreme 
anterior end of the medullary plate is quite flat, destitute of a 
median groove, and separated from the underlying proamnion by 
the head fold. It therefore consists of two layers, a thicker 
upper and a thinner lower ]a} r er, separated from each other by a 
narrow space (figs. 1, 15 and 16). 
Immediately behind the anterior end of the medullary plate its 
margins become upturned to form the medullary folds which are 
conspicuous in the photo-micrograph as the two dark lines on 
either side of the future fore-brain. A transverse section through 
the middle region of the fore-brain is shown in fig. 2. In the 
middle of the section is a well-marked groove (d. fr.) separated by 
elevations of the medullary plate from two lateral grooves (op. gr.) 
bounded externally by the medullary folds which curve slightly 
inwards above the grooves. The median groove will form the 
future first cerebral vesicle, while the lateral grooves we regard as 
the Anlagen of the future optic vesicles. Heape* figures a section 
very similar to our fig. 2 through the optic grooves of the Mole 
(Stage R)j and comments on their very early appearance, while 
the medullary groove is still widely open. Posteriorly each optic 
groove deepens, becomes somewhat V-shaped in section, and at 
the same time its floor thickens, eventually forming a hollow out- 
growth which arises from the bottom of the groove and proceeds 
outwards and downwards (fig. 3). An appearance similar to this 
has not been described, so far as we are aware, for any other 
embryo, and from the evidence at our disposal it would be rash 
to speculate too far as to its probable significance. The whole 
Quart. Jour. Micros. Science, Vol. xxvii. 1887, fig. 16, PI. xi. 
