200 KINGSLEY AND CONX 



As time passes the blastoderm gradually extends itself over the surface of the 

 yolk until, at a later stage than that described in this section, it completely embraces it, 

 thus forming the yolk sac. When about one-fourth of the surface of the yolk was thus 

 covered, the segmentation cavity was first noticed. No observations were made regarding 

 its mode of origin but it was doubtless by a lifting up of the blastoderm. In the earli- 

 est stages its roof was formed by the epiblast alone, its walls of lower layer cells while its 

 floor was formed by the yolk, or rather by the intermediary layer which rests upon the 

 yolk. The floor at this time was perfectly free from nuclei or cells. At first the segmen- 

 tation cavity is low, circular in outline, with its lateral margins about equidistant from the 

 edge of the blastoderm. The blastoderm continues its extension over the yolk and 

 increases rather more rapidly in one position of the margin than on the others, and at the 

 same time the lower cells encroach upon the segmentation cavity from one side until the 

 cavity becomes eccentric and is placed nearer one portion of the blastodermic margin than 

 to the others. This pushing in of lower layer cells continues until the cavity acquires an 

 arcuate or reniform outline. At this time free cells are numerous upon the floor of the 

 cavity. 



This is the first opportunity we have for the orientation of the egg. The segmentation 

 cavity is farthest from the portion of the blastodermic margin where the first outlines of 

 the germ are to appear and so we may now speak of anterior and posterior portions of the 

 blastoderm, the segmentation cavity is anterior and the embryonic area posterior. 



The invagination of the hypoblast now begins. As before stated we were unable to cut 

 actual sections but the extreme transparence of the eggs rendered this almost a superfluity. 

 Our observations on the invagination were made both by surface views and by optical sec- 

 tions, the latter being in almost every respect equal, to actual ones while from the fact that 

 the steady progress of the invagination could be continuously watched in the living egg 

 they presented advantages which no product of the section knife could equal. 



At all points of the margin of the blastoderm a single layer of cells may be seen push- 

 ing themselves inward beneath the rest of the blastoderm and separated from the lower 

 layer cells by a well defined line. Between this hypoblast and the yolk is still to be 

 found the intermediary layer. The invagination progresses much more rapidly from the 

 posterior or embryonic portion of the blastoderm than from any other portion of its mar- 

 gin, and at the anterior portion more rapidly than at the sides. Fig. 22, pi. xiv, rep- 

 resents an optical section on the median line of an egg in which invagination has just 

 begun, while fig. 23 represents the same at a somewhat later stage. In this last figure 

 the extent of the lateral invagination is shown by the shaded area. Fig. 24 gives a view of 

 the lower surface of the same blastoderm showing the rates at which the invagination pro- 

 gresses in different parts. (The dotted line indicates the plane on which the section 

 described is taken.) The invagination continues until it forms a layer entirely separating 

 the rest of the blastoderm from the yolk and intermediary layer. Its later stages and 

 the phenomena accompanying it belong more properly to the next section. 



Our attention was not especially directed toward the origin of the mesoblast but we are 

 of the opinion that it arises partly from the lower layer cells and partly from the hypo- 

 blast. Whether it arises as two lateral plates, we know not, but at an early stage it forms a 

 continuous layer extending across the embryonic area as shown in fig. 25. With 



