BLOOD GLANDS 343 



of the peritoneal investment of that organ. Thus, in the Dog- 

 Fish (ScyUium), the spleen is a large reddish body attached to 

 the convexity of the U-shaped stomach, and, in addition, sends 

 a long narrow lobe between the distal limb and the valvate 

 portion of the intestine (Pig. 153, spl). In the Sturgeon 

 (Aeipenser), the organ is also large, but is attached to the left 

 side of the commencement of the intestine. In the Cod (Gadus) 

 among Teleosts the spleen is much elongated and is situated 

 on the dorsal side of the stomach. In the Dipnoi (e.g. Proto- 

 pterus) 1 the organ is probably represented by a large compact 

 lymphoid mass, closely connected with the dorsal and lateral 

 walls of the stomach (Fig. 154, A, s). 



The Thyroid Gland. — This organ " usually arises in the form 

 of a small median evagination of the hypoblastic epithelium of 

 the ventral wall of the pharynx, in the region of the second 

 visceral arch. Later it becomes detached from the place of 

 origin and converted into a solid spherical body. Eventually 

 the component cells form the limiting epithelium of a series of 

 follicles or vesicles embedded in a matrix of connective tissue and 

 blood-vessels, and the characteristic adult structure is attained. 



Among the Cyclostomata the evagination is relatively large 

 in the young Lamprey {Petromyzon fluviatilis), as also is the 

 orifice of communication with the pharynx (Fig. 202, th)? The 

 aperture soon becomes reduced to a mere pore, and finally 

 disappears. During the larval or Ammocoetes-stage the organ 

 consists of a median cilated portion, communicating with a pair 

 of laterally placed glandular sacs, but in the adult it is much 

 smaller, and acquires the usual follicular structure. In adult 

 Elasmobranchs the thyroid is represented by a moderately large 

 compact organ, situated near the anterior end of the ventral 

 aorta. In Teleostomi the organ may be paired, or, as in the 

 Perch [Perca), more diffuse, consisting of masses of reddish 

 lobules lying beneath the aorta, and also scattered for a variable 

 distance along the course of the afferent branchial arteries. 



In the Dipnoi (e.g. ProtopterusY the thyroid is small, con- 



'■ Newton Parker, op. cit. p. 138. 



2 De Meuron, Seeherches sur U diveloppement du Thymics et de la cjlande thyrmide, 

 Inaug. Dissert. Genfeve, 1886 ; Maurer, Morph. Jahrb. xi. 1886, p. 129 ; W. Miiller, 

 Jen. Zeitsch. vi. 1871, p. 428 ; vii. 1873, p. 327 ; Dohrn, Mitth. Zool. Stat. Neapel, 

 vi. 1886, p. 49 ; vii. 1887, p. 301. 



3 Qf_ p^ 280. " Newton Parker, op. cit. p. 135. 



