442 SUMMARY OF CURRENT RESEARCHES RELATING TO 



entire extent except in the iris ; pigment extended into the inner layer 

 of the secondary optic cup ; there were no ciliary processes. Had the 

 embryo grown to maturity, it would have had a large coloboma of the 

 iris, choroid, and retina. 



American Alligator.* — A. M. Reese gives a general outline of the 

 whole process of development in the American Alligator (A missis- 

 sipiensis), which has hitherto been little studied owing to the difficulty 

 of procuring suitable embryological material. Owing to the fact that 

 the embryo may undergo considerable development before the egg is 

 laid, and also to the uuusual difficulty of removing the young embryos, 

 the earlier stages of development are very difficult to obtain. The 

 mesoderm seems to be derived chiefly by proliferation from the endo- 

 derm, and in this way all that is anterior to the blastopore arises. 

 Posterior to the blastopore, the mesoderm is proliferated from the lower 

 side of the ectoderm in the usual way. No distinction can be made 

 between the mesoderm, derived from the ectoderm and that derived 

 from the endoderm. The ectoderm shows, during the earlier stages, a 

 very great increase in thickness along the median longitudinal axis of 

 the embryo. The notochord is apparently of endodermal origin, though 

 in the posterior regions where the germ-layers are continuous with each 

 other it is difficult to decide with certainty. The medullary folds have 

 a curious origin, difficult to explain without the use of figures. They 

 are continuous posteriorly with the primitive streak, so that it is impos- 

 sible to tell where the medullary groove ends and the primitive groove 

 begins, unless the dorsal opening of the blastopore be taken as the 

 dividing point. The amnion develops rapidly, and entirely from the 

 anterior end. The blastopore, or neurenteric canal, is a very distinct 

 feature of all the earlier stages up to about the time of the closure of 

 the medullary canal. Preceding the ordinary cranial flexure, there is a 

 sort of temporary bending of the head region, due apparently to the 

 formation of the head fold. During the earlier stages of development 

 the anterior end of the embryo is pushed under the surface of the 

 blastoderm, and hence is not seen from above. Body torsion is not so 

 definite in direction as in the chick, some embryos lying on the right 

 side, some on the left. 



Of the gill-clefts, three open clearly to the exterior, and probably a 

 fourth also. A probable fifth cleft was seen in sections, and in one 

 surface view. The first trace of the excretory system is seen as a dor- 

 sally projecting solid ridge of mesoblast in the middle of the embryo, 

 which ridge soon becomes hollowed out to form the AVolffian duct. The 

 origin of the pituitary and pineal bodies is clearly seen ; the latter 

 projects backwards. No connection can be seen between the first rudi- 

 ments of the sympathetic nerves and the central nervous system. The 

 lumen of the oesophagus is for a time obliterated as in other forms. 

 The choroid fissure is a very transitory but well-marked feature of the 

 eye. 



Embryos in Ichthyosaurs.f — W. Branca maintains that some of 

 the embryos found in Ichthyosaurs were swallowed. Inside the 



* Smithsonian Miscellaneous Collections, li. (1908) 66 pp., 23 pis. 

 t SB. k. Preuss. Akad. Berlin, 1908, pp. 392-6. 



