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CLARKE. [Vol. V. 



1 6 and 18. They were made from very perfect specimens, and 

 the sections of both of them, and of the specimen from which 

 Fig. 20 was drawn, proves that the structure is what it is indi- 

 cated to be in surface appearance. That is, the transverse sec- 

 tions posterior to the V, in the embryos shown in Figs. 16 and 

 1 8, show the medullary groove and the medullary folds: the 

 several sections passing through the apex of the V show neither 

 groove nor folds, but only a median thickening ; and in front of 

 the point or apex of the V the successive sections discover a 

 gradually widening groove between the arms, which is also much 

 deeper than the shallow groove found posterior to the V. While 

 I have not seen, and from the nature of the conditions one can- 

 not see, the change actually proceeding from the form of Fig. 18 

 to that of Fig. 20, still the explanation given appears to be the 

 only one possible. The next pair of figures, 21, ventral, and 

 22, dorsal, are of an embryo out of an egg five days after it was 

 taken from an alligator. The head-fold is increasing, and with 

 it the amnion ; three pairs of mesoblastic somites have been 

 formed ; the medullary folds are most widely separated anteriorly 

 in the region of the brain ; the blastopore still persists. The 

 large, light-colored spots in the opaque area are produced by 

 the separation of the germ layers from each other, forming a 

 blister-like structure. A somewhat later stage is shown in 

 Figs. 23, dorsal, and 24, ventral. The medullary folds are clos- 

 ing at the anterior extremity and in the middle region, but are 

 widely open just in front of the mesoblastic somites, and at the 

 posterior end, where they are surrounding the blastopore. One 

 more pair of somites has been added. These somites are added 

 posteriorly, as one may determine by comparing Figs. 21, 24, 

 and 25 ; and in longitudinal section one finds them in process 

 of formation immediately behind the last pair. In Figs. 25 and 

 26, ventral and dorsal of the same embryo, two pairs more have 

 been added. All the figures on Plates II and III, with the excep- 

 tion of 27 and 28, are from alcoholic specimens. Figs. 27 and 

 28 are from life, and are also a dorsal and ventral view of one 

 embryo. The parts show less clearly in the living embryo, and 

 all the somites present were not visible from the dorsal side. 

 The two prominent circular outlines on either side of the hind- 

 brain and neck region in Fig. 27 are portions of the edge of the 

 head-fold which show through from the ventral side. The 



