214 



THE ORIGIN OF VERTEBRATES 





may contain mucin in the younger developmental stages, and there- 

 fore possible that they might at that stage secrete it ; they certainly, 

 however, show no sign of doing so in their more adult condition, and 

 cannot be compared in the very faintest degree to the glandular cells 

 of the pharyngeal region. It is also perfectly possible for gland-cells 

 belonging to a retrograde organ to become mucus-secreting, and so to 



give rise to the cells of Am- 

 phioxus and the Tunicata. 



If, then, these cells were 

 not retained for digestive 

 purposes, what was their 

 function ? To answer this 

 question we must first know 

 the function of the corre- 

 sponding gland-cells in the 

 uterus of the scorpion, which 

 undoubtedly secreted into 

 the cavity of the uterus and 

 took some part in connection 

 with the generative act, and 

 certainly not with digestion. 

 What the function of these 

 cells is or in what way they 

 act I am unable at present 

 to say. I can only suppose 

 that the reason why the 

 thyroid gland has persisted 

 throughout the vertebrate 

 kingdom, after the genera- 

 tive tissues had found a new 

 outlet for their products in the body-cavity of the posterior region, 

 is because it possessed some important function in addition to that 

 connected with the exit of the products of the generative organs ; a 

 function which was essential to the well-being, or even to the life of 

 the animal. We do not know its function in the scorpion, or the 

 nature of its secretion in that animal. We know only that physiology 

 at the present day has demonstrated clearly that the actual external 

 secretion of a gland may be by no means its most important function ; 

 in addition, glands possess what is called an internal secretion, viz. a 



Fig. 93.— A, Portion op a Gill op Ammo- 



C03TES WITH ORDINARY RESPIRATORY EPI- 

 THELIUM ; B, Corresponding Portion of 

 the First or Hyoid Gill. 



