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 i3rimitive 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 layer, 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 sve 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 F.), and comments on their very early aj^pearance, while 

 the medullary groove is still widety open. Posteriorly each optic 

 groove deepens, becomes somewhat Y-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 an}^ other 

 embryo, and from the evidence at our disposal it would be rash 

 to speculate too far as to its jjrobable significance. The whole 



* Quart. Jour. Micros. Science, Vol. xxvii. 1887, fig. 16, Pi. xi. 



