214 E. H. NORRIS 



the cavities described in this paper. Since finding these intra- 

 glandular spaces in the thyreoids of human embryos and in those 

 of Squalus acanthias embryos, I have made a brief stud^f of 

 a number of intermediate forms (pig, sheep, dog, pigeon) and 

 have found similar cavities present in the thyreoid of each form 

 investigated. It is hoped that these observations may be ex- 

 tended and presented in a subsequent pubhcation. The existence 

 of this same feature of morphogenesis in the thyreoid glands of 

 two animal groups as far removed from one another as fish and 

 man, as well as in the glands of a number of intermediate forms, 

 will probably justify the conclusion that this is a fundamental 

 feature of thyreoid development. 



Ajiy attempt to interpret these early intraglandular spaces 

 either on the basis of their immediate or general biologic signifi- 

 cance, although interesting, can be at best only speculative. It 

 might be thought that the immediate purpose and significance of 

 these cavities is to be found in the formation of the two-celled 

 plates from which the follicles are later to be derived. On the 

 other hand, if the vertebrate thyreoid is the phylogenetic repre- 

 sentative of a true externally secreting gland (Patten, '17), it 

 might be suggested that these cavities appear in response to a 

 tendency to reproduce the ancient lumen or duct of the ancestral 

 gland. This theory is attractive, inasmuch as it permits of 

 harmonizing the known facts, both anatomical and physiological, 

 regarding the endocrine function of the thyreoid. From this 

 point of view the hypothesis and conclusion of Bensley ('16), 

 'that the thyroid cell represents a true reversal of polarity," 

 would be quite unnecessary. On the other hand, this phylo- 

 genetic theory would offer a plausible explanation of the present 

 structure of the gland and its endocrine function without making 

 it essential to hypothesize a reversal of cellular polarity. 



It might be objected that the fact that these intraglandular. 

 spaces have no relation to the lumen of the thyreoglossal duct 

 would appear to be a serious obstacle to the theory just advanced. 

 But such a contention is not as formidable as it might seem at 

 first sight, when it is considered that in the form at present under 

 discussion no thyreoglossal duct is ever formed, and, moreover, 



