8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



mass of cells. Laterally the mesoderm (mes) is a distinct layer of 

 cells of a fairly characteristic mesodermal type. The notochord is 

 not yet discernible, though a slight condensation of cells in the middle 

 line may indicate its position. 



Figure 2/ is one of the four sections that were cut through the 

 blastopore (blp), which is a hole of considerable size that opens, as 

 the figure shows, entirely through the blastoderm. Along the walls 

 of the blastopore the ectoderm and entoderm are, of course, contin- 

 uous with each other and form a sharply defined boundary to the 

 opening. As we pass laterally from the blastopore the cells become 

 less compact, and are continued on each side as the mesodermal layer 

 (mes). In this series the sections posterior to the blastopore were 

 somewhat torn, and so were not drawn ; but they probably did not 

 differ materially from those of the corresponding region of the im- 

 mediately following stages, which are shown in figures 3m and 6i 

 and will be described in their proper order. 



Stage II 

 Figures 3-30 (Plates II, III, IV) 



The next stage to be described is shown in surface views in figures 

 3 and 3a. Of this stage Clarke says : 



"The head-fold rapidly increases in depth and prominence, as 

 shown in figure 3, which is a ventral view a few hours later [than 

 the preceding stage]. The time cannot be given exactly, as it is 

 found that eggs of the same nest are not equally advanced when laid, 

 and differ in their rate of development. The lighter curve in front 

 of the head-fold is the beginning of the anterior fold of the amnion. 

 The notochord has been rapidly forming, and now shows very dis- 

 tinctly on the ventral side, when viewed by transmitted light. A 

 dorsal view of the same embryo (fig. 3a) shows that the medullary 

 or neural groove is appearing, and that it ends abruptly anteriorly 

 near the large transverse head-fold. Posteriorly it terminates at the 

 thickened area in front of the blastopore, which still remains open." 



Figures 3&-W are drawn from transsections of an embryo of about 

 this state of development. For a short distance in front of the be- 

 ginning of the head-fold, there is a mass of cells of considerable 

 thickness between the ectoderm and entoderm. In figure 36 these 

 cells appear as an irregular thickening of the entoderm, while in fig- 

 ure 3c they form a continuous mass, uniting the upper and lower 

 germ layers. This condition is seen, though in a much less striking 

 degree, in the following stage of development. As to its significance 

 the writer is not prepared to decide. 



