XII BLOOD GLANDS 343 
of the peritoneal investment of that organ. Thus, in the Dog- 
Fish (Sey/lium), 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 (Fig. 153, spl). In the Sturgeon 
(Acipenser), 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)' 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, ti). 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 (Perea), 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. Protopterus)* the thyroid is small, con- 
1 Newton Parker, op. cit. p. 138. 
2 De Meuron, Recherches sur le développement du Thymus et de la glande thyreotde, 
Inaug. Dissert. Genéve, 1886 ; Maurer, Morph. Jahrb. xi. 1886, p. 129; W. Miller, 
Jen. Zeitsch. vi. 1871, p. 428 ; vii. 1873, p. 327 ; Dohrn, Mitth. Zool. Stat. Neapel, 
vi. 1886, p. 49; vii. 1887, p. 301. 
3 Cf. p. 280. 4 Newton Parker, op. cit. p. 135. 
