444 MORGAN. [Vol. X. 



in the germ-ring and lateral ring of the embryo is formed of 

 polygonal cells that appear normal. 



Another embryo is drawn in Fig. 33. Here the elongation 

 of the embryo, and the closure of the blastopore, has taken 

 place more normally than in the last cases. This embryo also 

 shows, with great distinctness, that on each side of the neuro- 

 notochordal plate, a band of tissue is directly continuous into 

 the germ-ring, and we see this tissue is also distinctly separated 

 from the central neuro-notochordal plate. 



It is not at all uncommon to find embryos that show the 

 condition seen in Fig. 35. The tail-knob projects beyond the 

 meeting point of the germ-ring, i.e., the tail-knob projects into 

 the blastopore region. 



Another such modification is shown in Fig. 36, where the 

 protrusion of the tail-knob is greater than in the last. The 

 germ-ring is attached further up on the left side of the embryo 

 than on the right. This embryo was cut into serial cross-sec- 

 tions. The sections show that the ectoderm destined to form 

 the nervous system has not concentrated along the axial line 

 but is spread out in a broad, flat plate over the posterior dorsal 

 surface of the embryo. In the middle line, beneath the flat 

 plate, the rounded notochord is perfectly formed. The meso- 

 derm forms two thick bands along each side of the notochord, 

 and is composed of polygonal cells. 



The sections through the posterior end of this embryo show 

 that the lateral sheets of mesoderm continue into the under 

 layer of the germ-ring, and it seems, moreover, that it is 

 only the outermost portions of the mesoderm of the sides that 

 go into the germ-ring, so that a small amount continues, for a 

 short distance along the sides, posterior to the point of union 

 of the germ-bands. It looks, from the evidence furnished by 

 these sections, as though the mesoderm of the germ-ring laid 

 itself down along the sides of the mesoderm of the body. 



The protrusion beyond the level of the union of the germ- 

 bands is the tail-knob, in which the three (.'') layers are not 

 sharply separated from one another. 



Another point of interest is seen in this series of sections. 

 The ectoderm above the germ-ring near to the embryo is 



