PERSISTENT CELLULAR PARENCHYMA. PLACENTAL CELLS. 



151 



transformation, but which form part of the substance of the fabric, instead of 

 lying upon its free surfaces and being continually cast oft' from them. Still 

 their individual history is much the same as that of the cells already noticed ; 

 and they differ chiefly in regard to the destination of their products. There 

 are many animals, in which such aggregations of cells make up a much larger 

 part of the fabric, than they do in Man ; and this in consequence of their re- 

 taining more of the embryonic type of structure in their adult condition. 

 Thus in the Myxinoid family of Fishes, there is no true Vertebral column : 

 but its place is supplied by a gelatinous tube, termed the chorda dorsalis ; 

 which consists of nucleated cellular tissue, and which is precisely analogous 

 to the structure occupying the same position in the early embryo of higher 

 animals. In the Short Sunfish, a corresponding form of tissue forms a thick 

 covering to the body, replacing the true skin. And in the Lancelot (a little 

 Fish which is deficient in so many of the characters of the Vertebrated di- 

 vision, that many naturalists have doubted its right to a place in the class), a 

 considerable portion of the fabric is made up of a like cellular parenchyma. 



181. The first group of this class deserving a separate notice, is that which 

 effects the introduction of aliment into the body; of those kinds of aliment, 

 at least, which are not received in solution by any more direct means. These 

 cells (first pointed out by Mr. J. Goodsir) form a cluster at the extremity of 

 each of the villi of the intestinal tube ; the origin of the lacteal being lost in 

 the midst of it. If examined whilst the absorbent process is going on, they 

 are found to be turgfid with a milky fluid, which is evidently the same with 

 that of the lacteals ; and to have a diameter of from l-2000th to l-1000th of 

 an inch (Fig. 46, A). In the intervals of the digestive process, the extremities 

 of the villi are comparatively flaccid : and instead of cells, they show merely 



Fig. 46. 



Extremity of intestinal villus; seen at A, during absorption, and showing absorbent cells and lacteal 

 trunks, distended with chyle ; at B, during interval of digestion, showing peripheral network of lacteals, 

 with granular germs of absorbent cells, as yet undeveloped, lying between them. 



a collection of granular germs (Fig. 46, b). These begin to develope them- 

 selves, as soon as the food has been dissolved in the stomach and transmitted 

 to the intestine ; and their development goes on so long as they are surrounded 

 with nutrient matter. The cells grow, select, absorb, and prepare the nu- 

 tritious matter, by making it a part of themselves ; and, when their work is 

 accomplished, they deliver it to the lacteals by their own rupture or deliques- 

 cence, at the same time, it is probable, setting free the germs, from which a 

 new generation maybe developed, when the next supply of chyle is prepared. 

 182. Although the mucous membrane of the intestinal tube is the only 

 channel, through which insoluble nutriment can be absorbed in the completely- 

 formed Mammal, and the only situation, therefore, in which we meet with 

 these absorbent cells, there are other situations, in which similar cells perform 

 analogous duties in the embryo. Thus, the Chick derives it nutriment, whilst 

 in the egg, from the substance of the yolk, by absorption through the blood- 

 vessels, spread out in the vascular layer of the germinal membrane that sur- 

 rounds it ; which vessels answer to the blood-vessels and lacteals of the per- 



