STUDIES ON THE SPINAL CORD AND :MEDULLA OF 

 CYCLOSTOMES WITH SPECIAL REFERENCE TO 

 THE FORMATION AND EXPANSION OF THE ROOF 

 PLATE AND THE FLATTENING OF THE SPINAL 

 CORD' 



WILLIAM F. ALLEN 



Deparlinent of Anatomy, University of Minnesota 



EIGHTY-SEVEN FIGURES 



CONTENT? 



I. Introduction it 



Material and its preparation 10 



11. Hereditary and mechanical causes underlying the formation of the 



fourth ventricle and the tela chorioidea 12 



1. Roof plate of the medulla oblongata 12 



A. Amphioxus 12 



B. Polistotrema (Bdellostoiun 18 



C'. Petromyzon la 



D. Selachians and Amphibia 23 



K. Pig embryos 25 



V. Human embryos 27 



2. Descriptions of three roof plate expansions of the spinal cord in the 



20 cm. Poli.stotrema series 28 



III. Causes underlying the flattening of the spinal cord in cyclostomes. ... 32 



IV. General considerations and sunmiary 35 



V. Literature cited 40 



INTRoDK'riOX 



This paper has grown out of the stud}' of the caudal heart and 

 the spinal cord and nerves related to it in Polistotrema. Parts 

 of that study which are not yet completed will be published 

 later and will deal with the origin, distribution, and phylogeny 

 of the spinal nerves; the origin of muscle sense organs in con- 

 nection with the specialized muscles of the caudal heart; and 



1 A thesis submitted in partial fulfilment of the requirements for the degree of 

 Doctor of Philosophy, Department of Anatomy. The University of Minnesota. 







