5oo 



HISTOLOGY 



the surface and discharged as a droplet, the vacuole from which it came 

 being closed behind it. 



These glands are probably specializations or phylogenetic derivatives 

 of sweat glands or of primitive glands from which the sweat glands were 

 also derived. This is shown by their simple epithelial lining and the 

 manner in which they produce both droplets of fat and a supply of the 

 watery constituents of the milk. 



The first part of this secretion, just after childbirth, is handled by 

 certain lymph cells, or amoebocytes, which crawl between the cells and lie 

 in the lumen. Here they receive the secretion and carry it out. They 

 are called the colostrum corpuscles. 



We must also consider here, certain embryonic membranes which 

 are used to take food material from a yolk or store of food instead of 

 from a maternal membrane. The parent is not concerned in this pro- 

 cess other than by the fact that she previously furnished the yolk and that 

 the process may take place in her body as well as outside of it. Such 

 cases are well, but narrowly, represented by the yolk membranes of a 



bird, the tern, and 

 those of a fish, the 

 toadfish. 



In the tern, the 

 development of the 

 embryo results in a 

 four-layered mem- 

 brane which stretches 

 from the body of the 

 young bird over the 

 yolk (Fig. 469). The 

 two inner layers of 

 the four develop a 

 system of capillary 

 circulation. This 

 plexus appears de 

 noi>o, and the blood 

 corpuscles appear as 

 a part of the tissue 

 that lie ( ?) within its 

 walls. The outer 

 layer is a simple epi- 

 thelium derived from 

 the ectoderm, and it has temporarily assumed the function of a respira- 

 tory membrane. 



The cells of the inner layer are very much specialized and are used 



FIG. 470. Section through an embryo of the toadfish, Opsanus. 

 pb.n., nuclei of the periblastic syncytium which elaborates yolk 

 for the use of the young fish, x 250. 



