130 KINETIC HORMONES — II 



glands hypertrophy and secrete at the time of implantation of the 

 embryo and during early stages of its development. Not only the 

 growth of the glands but also their secretion seem to be stimulated 

 by PROGESTERONE from the corpora lutea in the ovary. This 

 similarity to the hormones that are claimed to stimulate the 

 secretion of oviducal glands in the toad, Btifo, may lend weight to 

 the claim of progesterone to be concerned in the latter. It is clear, 

 however, that there are distinct differences between the hormone 

 pattern in the lower vertebrates and the mammals, and this line 

 of argument must be accepted with caution. 



4.13 MILK-SECRETING GLANDS 



AvES. In some birds, such as the pigeon, Columha, a nutritive 

 fluid, the so-called "pigeon's milk", is produced from the thickened 

 lining of the crop in both sexes and is regurgitated to feed the 

 unfledged young. This secretion seems definitely to be under the 

 control of prolactin from the adenohypophysis. The hormone is 

 effective even in castrated, hypophysectomized and adrenal- 

 ectomized birds (Turner, 1955); its action must therefore be 

 direct. 



Mammalia. Prolactin is also active in most mammals, induc- 

 ing the secretion of milk in the mammary glands; but it is only 

 able to act upon glands that have already been stimulated to grow 

 and to reach a certain level of size and development of the acini by 

 the combined action of oestrone and progesterone. It is not clear 

 whether the action of the prolactin, LSH, is direct, as in birds, or 

 whether the effect is due to its also stimulating the secretion of 

 progesterone. The former seems the more probable, but, if so, it is 

 the only example so far noted of a hormone that is able to stimulate 

 the secretion of both exocrine and endocrine glands. 



The action of prolactin is linked w ith that of other hormones in 

 that the muscular release of the milk from the glands into the 

 ducts, and so to the teats, is under the control of oxytocin 

 (§3.114). Other hormones must also have an effect upon milk 

 production through their effects upon metabolism, and in particu- 

 lar on the level of sugars, fluid and calcium in the blood (§§ 5.2, 

 5.3 and 5.4). 



Prolactin can apparently be inhibited by high levels of oestro- 



