m 



cells vary greatly in size , the largest having a diameter of over 

 0,020 mm. Below these cells and sharply bounded from them , are 

 to be seen the mucous layers, the layer immediately beneath the 

 bladder cells composed of cells with a diameter of 0,008 — 0,010 mm, 

 with nuclei 0,004 mm in diameter. Sections stained with picrocarmine, 

 showed the nucleus of these outer cells colored red , and the outer 

 thickened margin a faint yellow from the picric acid. Bits of skin 

 from diflPerent embryos of this age were taken from the following parts : 

 forehead, nostril, upper lip, shoulder, chest, back, abdomen, upper and 

 lower arm, palms and soles, umbilical cord, penis, buttocks, legs, nails 

 and feet. In all of these parts are to be found these peculiar outer 

 cells. In some places I could find them only in small and large clumps, 

 but the more carefully the sections were handled, the more of these 

 clumps were to be seen, giving one the impression of an external layer 

 of partially degenerated cells, some of which had already fallen off, 

 others being lost in the process of imbedding and manipulation. 



In embryos older than 4 months, the outer layer on most portions 

 of the body is seen only in places, small clumps and occasional bladder 

 cells being found. The latest period at which I have been able to 

 trace these cells over the entire body, was in an embryo of about 

 6 months. In embryos of 7 months, this layer has disappeared, except 

 in certain places that will be spoken of later. 



The most perfect specimens of the bladder cells are to be 

 found in the epidermal covering of the umbilical cord. The epider- 

 mal development here is, as has been pointed out by Dr. Minot^), 

 somewhat later than upon the general surface of the body. Em- 

 bryos of about 2 months show an ectoderm composed of a single 

 layer of cells, and at 3 months two layers are to be seen, of which 

 the upper is composed of cells already presenting the rounded, 

 domed, appearance of the bladder cells. In embryos between 3 and 

 4 months of age, there is a complete external layer of bladder cells. 

 In embryos of 5 and 6 months, however, only occasional clusters can 

 be seen, and at 7 months they have disappeared entirely, so far as 

 I have been able to determine. 



Pieces of epidermis, stripped where it was possible from the 

 corium , stained and examined with the outer surface uppermost, 

 showed that the bladder form was due to a swelling up of the central 

 portion of the cell. In well preserved specimens a continuous layer 

 of these large polygonal and bladder cells could be seen, and beneath 



1) Journal of Morphology, Boston, Vol.11, Nr. 3, April 1889, p. 381. 



