126 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 



(one can not in this case say which) of numerous rather large, rounded, intensely 

 staining granules, very similar to those described by Bonnet (1901) in the completion 

 plate of the dog. Although a few of these granules can be seen in other locations, 

 as also figured by Bonnet, they are by far most numerous and conspicuous in the 

 completion plate. 



The variations in size of the head process and completion plate, especially as 

 regard their breadth and the roughly corresponding variations in the lumen, are 

 shown in figure 4. Although such variations are recorded (cf. Rabl, /. c., Taf. iv), 

 they are unusually distinct and regular in this case. What significance may attach 

 to them we can not say. They seem too small to correspond with the future seg- 

 mentation of the mesoderm lateral to them, and we have been unable to discover any 

 special features in this mesoderm, such as possibly more active proliferation of cells 

 in relation to the enlargements, either opposite or between them. 



The mesoderm in the anterior half of the blastoderm is essentially the same 

 as that which we have already described; far anterior it becomes very thin. 



Any indications of a folding off of the embryo, of a proamnion, or bucco- 

 pharyngeal membrane are wanting. 



The structures thus far considered comprise the essential features of the embry- 

 onic anlage. Nowhere, as far as we can make out, is there any sign of future seg- 

 mentation, and nowhere in the embryo are there either blood-vessels or blood-cells; 

 but at the very anterior end of the embryonic disk there occur a number of prolonga- 

 tions of the exocrelom under the embryonic ectoderm. These exocoelomic diver- 

 ticula have a very small, distinct, but quite irregular lumen lined by cells similar to 

 those on the neighboring yolk-sac and amnion. They appear as rather long, 

 irregular, tubular ingrowths which take their origin from the exoccelom at the point 

 where the mesoderm of the amnion and yolk-sac meets the embryonic mesoderm. 

 The two anterior diverticula arise in the shallow groove under the anterior edge 

 of the blastoderm. Of these ingrowths there can be made out about four, two on 

 either side. The anterior pair, longer and more distinct, reach nearly to the middle 

 line. Of the posterior pair the right is very short, while the left runs parallel to and 

 just within the margin of the blastoderm. Judging from their location, they 

 might stand in some relation to the future pericardial co?lom. 



THE YOLK-SAC. 



Only approximate dimensions can be given here on account of the folding, 

 partial collapse, and a somewhat extensive tear near the anterior end of the sac. 

 We may estimate its antcro-posterior measurement at about 2.5 mm., in a dorso- 

 ventral line at about 2 mm., and a little less than this latter figure from side to side. 

 As seen in the illustrations (fig. 3; plate 1, fig. 3; plate 4, figs. 1 and 2) it is still very 

 large as compared with the embryo projecting well beyond it on all sides, partic- 

 ularly in front and on the right. Originally it was doubtless quite regular in shape. 

 The surface of the sac is for the most part quite smooth and regular, but over a 

 certain area on the fundus anteriorly it has the characteristic uneven, nodular 

 appearance arising from the early blood formation in this region. 



