Embryonic History 0/ Pteroplatsea micrura. 5 



and all that is now necessary is to draw attention to the slight 

 but significant differences which are observed in this earlier 

 stage while the embryo has still an ample fund of yolk to 

 draw upon — differences which enable us to picture the mode 

 of development of the milk-secreting elements. 



In the specimen under notice the nursing-filaments mainly 

 differ to the naked eye from those originally described in 

 being altogether smaller and in being uniformly distributed 

 like a coarse thick fur over the entire surface of the uterine 

 mucous membrane, instead of being restricted to certain 

 definite areas. 



Their average length is 11 millim. and their average width 

 about 1*25 millim. at the base and about 0'75 millim. near 

 the tip, and they are flat with a tendency to curl. 



When a trophonema is stained with carmine and examined 

 in glycerine as a flat transparent object, under a low magni- 

 fying power, the blood-vessels first attract attention. Running 

 in the margin, from base to apex on each side, is seen a 

 small artery which at the tip of the filament flows each into 

 its fellow, either in a single loop or, after" a single acute- 

 angled bifurcation, in a double loop, as shown in fig. 4. All 

 along its course this marginal arterial loop sends off from its 

 concavity numerous small branches, which form a dense super- 

 ficial capillary plexus with its long narrow meshes transverse 

 to the long axis of the trophonema ; and deeply beneath this 

 plexus, running up the middle of the trophonema in its basal 

 half only, is a spiral vein of some size. Higher magnifica- 

 tion shows that the surface of the trophonema is uniformly 

 covered with pavement epithelium, which dips down, but does 

 not become discontinuous, in the slightly excavated inter- 

 capillary meshes. 



A transverse section of a trophonema (fig. 5) shows at 

 either extreme the artery and near the middle the wider but 

 not very much wider vein cut straight across, the superficial 

 capillaries cut through in various planes, and at the circum- 

 ference of the section an unbroken ring of pavement epithe- 

 lium presenting slight depressions in many places between 

 the cut capillaries. Beneath the epithelium, stretching from 

 artery to artery but not round the arteries, on both faces of 

 the narrow section, is a long close line of pocket- or bulb- 

 shaped nests of cells, which in some cases are quite solid, in 

 other cases are hollowed out in the centre, and in yet other 

 cases form true acini " pointing," to use a surgical metaphor, 

 towards the superficial intercapillary denressions of the surface 

 epithelium above alluded to. 



It is unnecessary to go further into histological detail, since 



