THYREOID GLAND IN SQUALUS ACANTHIAS 



197 



soon increase in size and are thus brought more closely into 

 relation with one another. Nothing more can be said by way of 

 generalization regarding their size, shape, or arrangement, great 

 differences being found in these respects both in cavities of the 

 same gland as well as in glands in different stages of development. 

 These intraglaiidular cavities are not at all like the vacuole-like 

 spaces described above. In every case these cavities, unlike the 

 vacuole-like spaces, are outlined by a very distinct and sharp 

 margin, and in no case is there any visible content within the cav- 

 ity. It is important to note that these spaces are quite inde- 

 pendent of any external conditioris, and in no case in these early 

 stages do they^open to the outside of the gland. Furthermore, 



■•^-t^:-:^::;© ©£S>:^sig)|S'^($5\^^)^^ O' 





Fig. 5 Drawing of a cross-section through the thyreoid gland of an Acanthias 

 embryo 28 mm. long (No. 37). X 450. 



as shown in figures 5, 19, 20, 21, 22, they develop independently 

 of one another as isolated spaces and only secondarily may be- 

 come confluent. At a much later stage (28 mm.) these cavities, 

 for the first time, open to the outside and are invaded by blood- 

 vessels. These cavities develop only in the body of the gland 

 and have never been observed in the region of the tail (cauda) . 



Careful examination of sections of the gland in this period of 

 its development shows that this process of cavity formation is 

 accompanied or paralleled by — probably even preceded by — four 

 other processes, in all of which the cells actively participate. 

 The first of these is the beginning differentiation of cell bound- 

 aries, which originally are very faint and indistinct, but which 



