445 



cells covering the smaller cells below, and gave pictures not unlike 

 those represented by Dr. Minot in the human embryo (vid. sup.)- 



I have, besides, studied the epitrichium of the pig's embryo for 

 purposes of comparison. An embryo 8 cm in length exhibited an 

 outer layer of large cells, somewhat flattened , with oval nuclei, and 

 deeply stained, while below were two layers of the rete much smaller 

 and less prominent. In an embryo 9 cm in length was found this 

 same layer better developed. The appearances were strikingly like 

 those seen in lig, 1. The margins of the cells were thickened and 

 highly refractive, and the peculiar domed appearance was also seen 

 in these upper cells. In an embryo 18 cm long, an epitrichium was 

 found on the hoof, well developed. The specimens at this age at my 

 disposal were not, however, sufticiently perfect to justify me in drawing 

 any deductions from them. I hope to be able, from future work, to 

 demonstrate more completely still the homology of this layer in man, 

 with the epitrichium of animals. 



A further proof of the significance of these outer cells is to be 

 found in the epidermal structures covering and surrounding the foetal 

 nail. Unna's eponychium (meaning by this a partially horny tissue 

 under which the foetal nail develops, and homologous with the epi- 

 trichium covering the hoof in animals) is a part of our epitrichial 

 layer, and continuous with the bladder cells, which are so numerous 

 upon the palmar and plantar surfaces. While engaged in these studies 

 my attention was attracted by Zander's article on the development 

 of the nail ^), in which he describes in embryos of 13 weeks and over, 

 peculiar cell elements on the surface of the epidermis of the fingers 

 and toes. The cells he describes as having a firm, bright, glistening 

 margin, and possessed at first of a large circular or oval nucleus. 

 Besides these, more or less irregular forms, vesicular and bladder cells 

 are to be seen, and this bladder form Zander ascribes to a swelling 

 up of the cell in the liquor amnii. He thinks that these cells have 

 pushed their way up from the lower epidermal layers to the surface, 

 and have there died. Last year Kölliker in his latest paper on the 

 naiP) refers to these bladder cells mentioned by Zander, and gives 

 us for the first time the following interesting account of their appear- 

 ance and distribution. „Die Oberhaut junger Embryonen zeigt an- 

 fänglich noch keine Schüppchen, sondern besteht einfach aus rund- 



1) Archiv f. Anat. und Physiol. Anat. Abt., 1886, S. 273. 



2) Die Entwickelung des menschlichen Nagels. Zeitschrift f. wissen- 

 schaftl. Zoologie, Bd. XL VII, 1. Heft, 1888. 



