346 J. T. PATTERSON. 



as this which give rise to the condition frequently seen in whole 

 mounts, in which the surface of the polygerm displays many 

 protuberances. 



(/) Formation of the Secondary Masses. The primary masses 

 soon become broken up into secondary masses. This is also 

 brought about by constrictions of the nucleated membrane (Fig. 

 23). These secondary masses may contain more than one 

 embryo, in which case they immediately form constrictions which 

 result in producing still smaller masses, each of which contains a 

 single embryo. 



In the constrictions which lead to the cutting off of a single 

 embryo with its involucres, some of the precipitated material is 

 enclosed between that portion of the granular layer which is in 

 contact with the embryo and that part lying adjacent to the 

 inner surface of the nucleated membrane. These two parts of 

 the granular layer then fuse, forming a single involucre in which 

 are the spaces containing the precipitated material (Fig. 24). 

 The embryo is thus surrounded by two involucres, a granular 

 layer, and a nucleated membrane (Fig. 26). In some cases the 

 precipitated material may be so abundant as to form a solid zone 

 between the inner and outer parts of the granular layer; in others 

 it is small in amount and gives the appearance of much flattened 

 nuclei lying within this layer (Fig. 26, P.M.}. 



(g) The Inter-embryonal Substance. At the close of the for- 

 mation of the single embryonic masses and their involucres the 

 inter-embryonal interstices are already filled with a substance 

 derived from several different sources. It consists of a plasma- 

 like matrix in which are embedded cells and nuclei. We have 

 already noted that during the formation of the primary and 

 secondary masses some of the elements from the nucleated 

 membrane are not included in the outer involucre, but are left 

 in the inter-embryonal spaces. During the early history of the 

 inter-embryonal substance, it consists mainly of product from 

 this membrane. Later cells from two other sources enter into 

 its formation. First, leucocytes from the host are found 

 embedded in the matrix. They are especially abundant in those 

 regions of the polygerm exposed directly to the body cavity, 

 that is near a surface barren of adipose tissue. Second, fat cells 



