57 



alimentary canal and of the nervous system will be given on the fol- 

 lowing pages. 



THE DEVELOPMENT OF THE EXTERNAL FORM 



Figure 21, Plate V, represents an embryo about twelve to four- 

 teen days old. The embryo occupies a position somewhat nearer the 

 posterior than the anterior end of the egg, with a mass of large, 

 faintly-staining cells at each end. The serosa encloses these cells 

 wnth the embryo. The ventral thickening appears as a ridge along 

 the median ventral line, curving around at each end in the form of 

 a- large letter C. At the anterior end there is a slight widening of 

 this ridge which indicates the beginnings of the procephalic lobes. 

 At the posterior end the thickening passes insensibly into the darker 

 posterior dorsal portion of the embryo. This is due to the fact, as 

 we have noticed in the sections, that the posterior end of the original 

 germ-band breaks up into finger-like masses of small cells which 

 form a network around the large heavily-staining cells of the posterior 

 end of the blastoderm. A very slight indication of segmentation has 

 already made its appearance, due to the beginning of the formation 

 of the ganglia of the ventral nerve-chain, the thickenings of which 

 may be seen in lightly stained embryos. Sections of this stage show 

 that the invagination of the stomod?eum has just begun, but there is 

 no indication of it as one looks at the entire embryo. The invagin- 

 ation of the proctodasum has not yet begun. 



Figures 22 and 25 represent a stage in which the layer of cells 

 from the inner posterior dorsal part of the embryo has grown around 

 to the ventral side along its entire length, thus completing the mesen- 

 teron, which now has the appearance of a pear-shaped sac, closed at 

 both ends, the small end being the anterior one. This pear-shaped 

 sac enclosing the yolk almost fills the embryo, though a small space 

 is noticeable Ijetween it and the outer layer or ectoderm, this space 

 representing the body cavity. At the anterior end of the embryo 

 there is a well-developed, thick-walled, backward-projecting, U-shaped 

 invagination of the outer ectoderm which represents the stomodasum. 

 The posterior end of the stomodaeum almost reaches the anterior 

 end of the mesenteron. The invagination of the proctodcTitm is also 

 well-developed at this stage. At the anterior end of the embryo at 

 this stage the appendages are already well formed. Just in front 

 of the stomod.'eum is a median evagination of the ectoderm pro- 

 jecting anteriorly and dorsally, which when viewed from the side 

 appears as a pcnr-sliapcd body, but when A-iewed from a postero- 



