EVOLUTION OP ORGANS IN THE OHORDATA. 279 



embryo in the middle line very far forward as an outgrowth of 

 endoderm cells close behind the mouth, and subsequently 

 passes backwards losing its connection with the pharynx. In 

 a note Dohrn promises in a future study to discuss the spira- 

 cular cleft of the Selachians and Ganoids, and the pseudobranch 

 of Teleosteans, and to show that between the mandible and 

 the hyoid in Teleostean embryos on each side a deep invagina- 

 tion of the ectoderm occurs, which is to be regarded as the 

 ectodermal part of the cleft represented by the thyroid. (It is 

 probable that this invagination is the same as that observed by 

 other embryologists and diagnosed as the Teleostean represen- 

 tative of the spiracle.) In another note it is stated that 

 evidence will at a future time be adduced to show that in the 

 jaw and hyoid system of Teleosteans five independent visceral 

 arches are combined : 1, upper jaw ; 2, lower jaw ; 3, spiracular 

 cartilage ; 4, hyomandibular ; o, hyoid. 



The Thyroid of Petromyzon. 



The subject discussed in Study VIII is the thyroid in 

 Petromyzon and its homologue in Amphioxus and the Tuni- 

 cata. In the larval Ammocoetes the first trace of the thyroid 

 appears at the time when the most anterior branchial diver- 

 ticula of the endoderm grow out. Its first rudiment is a 

 diverticulum directed downwards and somewhat forwards, close 

 beneath the median part of the first pair of branchial diver- 

 ticula, which is homologous with the spiracular clefts of Sela- 

 chians and the pseudobranchiae of Teleosteans. Between the 

 stomodseum and enteron on each side runs the most anterior 

 branchial artery, homologous with the spiracular artery of the 

 Selachians ; it opens into the cephalic aorta of its own side, 

 Petromyzon possessing two cephalic aorta one on each side of 

 the notochord. The growth backwards of the mesoderm of 

 the velum causes the opening of the thyroid diverticulum to be 

 pushed farther back, so that it soon comes to lie at the level of 

 the second pair of branchial sacs, and later between the second 

 and third. A sagittal ingrowth of mesoderm now divides the 

 thyroid anteriorly into two halves. On each side another 



