328 MR FRANK BALFOUR-BROWNE ON 



has commenced. By careful focussing and manipulation of the light a median groove, 

 the " primitive streak," is to be seen, which extends for about half the length of the 

 egg from the posterior end. 



On the third day the groove is more visible, and at the posterior end of the egg the 

 commencement of the amnion and the amniotic cavity is apparent. There are also 

 indistinct traces of segmentation of the yolk, and twenty-four hours later these have 

 developed into complete segmentation. By this time — the fourth day — the amniotic 

 fold has extended forwards over the median groove, but has not yet completely enclosed 

 it, so that the blastopore is to be made out over certain irregular outgrowths which 

 constitute the beginnings of the embryonic head. The groove has extended round the 

 posterior end of the yolk and along the dorsal side so that the " tail" end of the 

 developing embryo lies almost over the head. Indistinct traces of somites are 

 visible in the embryo which, by transmitted light, is more transparent and less 

 granular than the yolk. 



On the fifth day the form of the embryo is quite distinct, the yolk having consider- 

 ably diminished. The embryo lies round the yolk, its ventral side towards the shell. 

 The appendages of the head, thorax, and abdomen are recognisable as buds. It is to be 

 noticed that the tail end of the embryo does not now extend quite so far round the yolk 

 as it did in the previous stage. 



On the sixth day the mouth parts, the maxillae, and the labium (which at present 

 consists of two separate parts) have folded on themselves and the maxillary palpi are 

 recognisable as buds. The embryo has still further contracted in length. The legs, 

 although recognisable, are at present only partly defined, but on the seventh day they 

 are more distinct. The segmentation of the abdomen, which on the sixth day has only 

 just commenced, is also more distinct on the seventh day, and the embryo is still shorter 

 in length. 



In the fifth-, sixth-, and seventh- day stages the embryo is seen to be surrounded by a 

 membrane— the amnion — within the shell. On the eighth day this has disappeared — it 

 having burst along the ventral side and shrunk back to lie as an irregular double ridge 

 on the dorsum. It goes by the name of "the dorsal organ" or "dorsal tube." The 

 embryo has now contracted so much that it lies straight out within the shell. At this 

 stage the eyes first become visible as six small pigment spots on each side of the head. 

 The labium is now a lilobed structure lying backwards on the ventral side of the head, 

 and the maxillae lie in the same direction on either side of it. 



During the next twenty-four hours the abdomen begins to lengthen out, but this 

 time it curves ventrally instead of dorsally. The legs, which up to now have lain close 

 together in the median line, have separated, leaving a space between those of the two 

 sides of the body. The "dorsal organ" has by the ninth day completely disappeared, 

 having sunk into the yolk and been absorbed. As soon as the dorsal organ disappears 

 the dorsum of the embryo closes completely over the yolk, which up to that time has 

 remained exposed. The trophi have lengthened and show traces of segmentation. 



