138 



HISTOLOGY 



P9- 



b. m. 



lum. or. 



FIG. 126. A first stage in the 

 development of a light- organ of 

 Porichthys. b.m., basement 

 membrane; mu.c., mucous 

 cells; pg.c., pigment cell in con- 

 nective tissue; lum.or., anlage 

 of luminous organ. (After C. 

 W. GREENE.) 



were separated from their parent tissue during the embryonic history by 

 the connective tissue. This process has been carefully described by 

 Greene in Porichthys. We shall study the 

 luminous organ of Porichthys. by tracing its 

 histogenesis, which is accessible and under- 

 stood. 



The skin of an embryonic Porichthys shows 

 a thin stratified epithelium much like that of 

 the shark. Of course no spines are present and 

 the mucous cells are somewhat numerous. 



At points in the basal layer of this epithe- 

 lium a crowding of the nuclei will be noticed, 

 and this is soon followed by an invagination 

 of the whole layer as well as a general thick- 

 ening of the epithelium at this point. Fig- 

 ures 126 and 127 show two invagination 

 stages in such a region after the invagination 

 has proceeded to some extent, and the structure resembles the light- 

 organ of Spinax except that its cells are not differentiated from one 

 another, and the outlying pigment cells, which are much increased in 

 size and number, have not moved into the epithelium. 



The process now proceeds farther than it did in Spinax by the con- 

 striction of the mouth of the in- 

 vagination, as in Figure 128, 

 where the rounded mass of cells 

 is separated from its parent epi- 

 thelium by the ingrowing con- 

 nective tissue. Here can also be 

 seen the beginning of a differen- 

 tiation of the proximal from the 

 distal cells of the mass. The 

 first are becoming granular and 

 vacuoles are appearing in their 

 cytoplasm. Pigment cells are 

 also present and usually more 

 numerous than in the specimen 



lum. or. drawn. They do not touch the 



FIG. 127. Later stage than Figure 126 of devel- invaginated mass but remain 

 sTm? H (After g c n w f oSS?T Le " ering *" constantly separated from it by 



a part of the connective tissue. 



This connective tissue also shows the beginning of a differentiation. 

 It forms plates and fibers which lie parallel with the proximal outline of 

 the invaginated cell mass and form a distinct layer. 



