Susanna Phelps Gage 411 



second arch great prominence while the third and fourth are in a common 

 depression, the sinus precervicalis. On the right the gill-clefts are less 

 crowded and the sinus not quite so deep. The first gill-cleft is wider at 

 its dorsal end indicating probably a beginning of the external meatus. 



The heart is prominent. The yolk sac is extensive turning to the right 

 instead of the left as is more usual, thus making the umbilicus more 

 apparent in a view from the right than from the left side (Figs. 5, 6). 



The limb-buds are remarkably prominent in comparison with other 

 specimens supposed to be of the same age. 



Epithelial thickenings. — In Fig. 1 are mapped out the regions having 

 a thickened epithelium. Details of these thickenings as shown by indi- 

 vidual sections are found in many of the figures, as the arm (Fig. 10, 

 11), the leg (Fig. 5), anal plate (Fig. 1), gill-arches (Figs. 1, 11), 

 mouth (Fig. 5), olfactory region (Fig. 2), lens (Figs. 1, 2, 5, 8, 16), 

 and the neuropore (Figs. 1-8, 16). In Fig. 1 the thickness of the 

 epithelium corresponds with the density of the dots. The portions left 

 white represent one layer of cells which become flattened over the heart 

 and near the dorsimesal line. Over the entire oblongata this layer of 

 flattened epithelium coalesces with the wide, thin roof of the brain (Fig. 

 4). Mall'' calls attention to the thickening of the neuropore in this 

 specimen. Especially noticeable is the H-shape of the thickening over 

 the olfactory and cerebral regions (Fig. 2). The continuity of leg-bud 

 thickening with that of the anal plate (Fig. 1) is comparable to the con- 

 dition in amphibian embryos. 



Age of the Specimen, 



The exact age of this specimen cannot be determined any more closely 

 than has been done by Dr. Mall, who considers that it is of about twenty- 

 one days development. Its relative age is somewhat important since it 

 presents certain features not hitherto fully described. It is necessary^ to 

 determine whether it may be a transitional stage or a pathologic or 

 arrested condition which is under observation. 



Though it presents one feature (the small number of thoracic myo- 

 tomes) not universal, and others (the umbilicus turning to the right) 

 which are not common, and still othei*s rare or not previously observed, 

 still on the whole, it so well fits into the series which has been described 

 by various authorities or examined by the author that the weight of evi- 

 dence seems to point in the direction that this is in general a normal 

 transitional condition, though possibly somewhat exaggerated in a few 

 particulars or retarded in others. 



