DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 25 



The head at this stage begins to push clown into the yolk in a 

 strange way that will be described later. 



Stage VIII 

 Figures 11-iiJb (Plates XI, XII, XIII) 



This stage is about one-fourth longer than the preceding. The 

 medullary canal is enclosed throughout its entire length, though it 

 appears in surface view (fig. 11) to be open in the posterior half 

 (mc) of the embryo. An enlargement of this apparently open re- 

 gion at the extreme posterior end (pg) is probably caused by the 

 remains of the primitive groove or the neurenteric canal, and a 

 slight opacity at the same point may be caused by the primitive 

 streak. The anterior end of the neural tube is bent in a ventral 

 direction (V), as in the preceding stage. The somites (s) now 

 number fifteen pairs ; they are somewhat irregular in size and shape. 



The head-fold is not so striking a feature as in the preceding 

 stage. The head-fold of the amnion (a) now covers nearly two- 

 thirds of the embryo. The heart (lit) is seen as a dark, rounded 

 object projecting to the right side of the neural canal, just anterior 

 to the first somite. The vitelline blood-vessels are just beginning to 

 form, but are not shown in the figure. 



The depression of the anterior region that was noted in the pre- 

 ceding stage has advanced so far that a considerable part of the 

 embryo now projects forward under the blastoderm. In some cases 

 it is almost concealed in a dorsal view ; in other cases it may easily 

 be seen through the transparent membranes, especially after clearing. 



In opening eggs of this stage one is at first apt to underestimate 

 the size of the embryos, since the anterior part of the embryos cannot 

 be seen until after they are removed from the yolk and are viewed 

 from the ventral side. 



The embryo from which the series of transverse sections of this 

 stage was made, while of the same state of development as that 

 shown in figure 1 1, was more fully covered by the blastoderm than is 

 shown in the surface view in question. 



Figure 11a passes through the tip of the head. Dorsal to the 

 embryo is the ectoderm and a thick mass of yolk (y). The amnion 

 (a) is seen as an irregular membrane which entirely surrounds the 

 head. The medullary canal (mc) is entirely closed, except at the 

 extreme anterior end, which is bent downward so that the opening 

 is on the ventral side. The nervous (nl) and epidermal (ep) layers 

 of the ectoderm are in contact throughout, but are clearly distin- 

 guishable because of the difference in the compactness of their cells. 



