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HISTOLOGY 



The relation is diffuse in some few animals (and most plants). This 

 diffuse connection by proximity can be well seen in the pollen sac of 

 Magnolia, where the outer members of a homogeneous cord of cells be- 

 come the nurse cells and feed the inner cells of the same mass, which 

 become the pollen cells (see Fig. 383). These pollen cells are formed 

 in essentially the same way as are animal spermatozoa. 



The diffuse relation between the source of nourishment and the nour- 

 ished spermatids, somewhat as indicated in the above example, is the 

 most general condition among lower, and especially the smaller, inverte- 

 brate animals. 



Good concrete examples of a more specialized connection are 

 to be seen in the vertebrate animals. Here the spermatids, soon 

 after their development into spermatozoa is begun, form an attach- 

 ment with a cell which lies near the basal layer of the reproductive 

 epithelium. 



They are drawn or move down between the other cells, and the head, 

 of each one of a large group, becomes partly embedded in and firmly 

 attached to the cytoplasm of this nurse cell which, in the mammals, is 

 called a Sertoli cell. Here they remain until maturity, when they are 



released and set free to pass out 

 of the reproductive lobule into the 

 organs used for then- distribution. 

 This will be described in con- 

 nection with the spermatogenesis 

 of the skate. 



In the salamander, Desmogna- 

 thus fusca, a somewhat different 

 method is used. As in the mam- 

 mal, the spermatids attach them- 

 selves to the nurse cell, but the 

 very large nurse cell is set free 

 toward the end of the sperm de- 

 velopment and floats about in 

 the lumen of the lobule, feeding 

 the spermatids until they are ripe. 

 It then degenerates and is lost, 

 freeing the spermatozoa. Figure 

 382 shows this condition. 



The first series of developing 

 reproductive cells that we shall 

 study will be in a plant, Magnolia 

 soulangeana. These asexual re- 

 productive cells only indirectly give rise to male reproductive cells. 



FIG. 382. Two sperm nurse cells from the sala- 

 mander Desmognathus fusca . Each n urse cell 

 has a group of half-developed spermatozoa at- 

 tached and is feeding them during their growth. 



