160 MANUAL OF HISTOLOGY. 



subject was first treated with, effect, were made by PurkinJQ and Valentin, comp. 

 Dephcenomeno generali et fundamentali motus vibratorii continui in membranis cum 

 externis turn internis animalium plurimorum et superiorum et inferiorum ordinum 

 obvii comment, phys. Vratislavice, 1835. 2. According to Englemann's varying 

 observations, this swinging of the cilia depends upon two motions of unequal length, 

 upon a longer one produced by the contractility of the protoplasm, and a shorter one 

 caused by elastic resistance. Currents of fluid passing over ciliated surfaces do so in 

 the direction of the last of these, in which also the cilia stiffen after death. 3. 

 Under certain circumstances, difficult to explain, the ciliary motion may persist for one 

 or .two clays after the death of a mammalian animal. 4. As Koelliker has demonstrated, 

 the same peculiarity is displayed by the spermatozoa. 



,98. 



Now, as to the origin of epithelium in the embryo, we must enter here 

 at somewhat greater length upon the consideration of the relations of 

 parts briefly touched on already at 86, in order to obtain a clear con- 

 ception of its development. 



As we have already seen, the rudimentary embryonic body according 

 to Remak (1.) consists of three layers of cells, of the so-called leaves or 

 germinal plates. These are known as the superior or the corneous, the in- 

 termediate or middle germinal, and the inferior or intestinal glandular 

 leaf or plate. From these the various tissues and organs of the body take 

 their rise. 



The corneous plate produces, first of all, the epithelium, with the nails 

 and hair which are closely allied to it, and beside these the crystalline 

 lens, a decidedly epithelial organ. The cellular elements, likewise, of the 

 various glands of the skin, together with those of the mammary and 

 lachrymal organs, take their origin from the same layer. Finally, the 

 axial portion of the corneous plate enters into the construction of the 

 nervous centres (brain and spinal cord) as well as the internal portions of 

 the higher organs of sense. That the peripheral nerves also originate in 

 the axial part of the corneous plate primarily, is at least probable. (2.) 

 The significance, consequently, of the corneous plate is very great, physio- 

 logically the highest in the body. 



Thus a large part of the epithelium described in former sections, the 

 epidermis, including those layers of cells which clothe the openings of the 

 larger passages of the body, takes origin from this source, and appears as 

 laminated epithelium, with a horny substance devoid of vitality. The 

 pigmentary pavement epithelium also, of the eye, together with the 

 internal coating of the cavities of the nervous system, are also derived 

 from this superior plate. 



The second, or intestinal-glandular plate, supplies the epithelia of the 

 digestive apparatus, as well as the cellular constituents of all the glands 

 in connection with the latter, including the lungs, liver, and pancreas. 

 Its epithelial formations appear principally in the form of the cylinder- 

 cell, either naked or ciliated. 



We must now finally devote a few moments to the middle germinal 

 plate, and inquire after its contributions to the epithelia. This middle 

 stratum of the rudimentary embryos supplies material for a great many 

 structures. First of all, for the formation of all the tissues of support in 

 the system, the whole group of connective* substances; for the building up 

 of muscle ; for the blood and lymph, together with the so complicated 

 system of vessels which contains both ; and finally, for the so-called 

 lymph or blood-generating glands (including the spleen). The cutis 



