646 Comparative Morphology of Chordates 



tion of endocrine substances (hormones) which play an important 

 part in the regulation of activities of the reproductive organs and 

 mammary glands. It is possible that it is this secretory function of the 

 ovary which makes it necessary for the large mammal to have a cor- 

 respondingly large ovary. In the present incomplete state of our 

 knowledge of the matter, there is room for the speculation that, in 

 both the evolutionary and the ontogenetic development of animals, 

 there is some factor whose operation tends to maintain a specific scale 

 of proportions of parts, irrespective of functional necessity — some- 

 what as in large modern buildings there may be massive columns which 

 play no important part in supporting the reinforced concrete structure 

 which appears to rest upon them. 



Two mammals, Ornithorhynchus and Echidna, are oviparous. Their 

 eggs are relatively enormous, the longer diameter, including the shell, 

 being 15 to 17 mm. The diameter of the actual ovum — i.e., the egg 

 exclusive of its envelopes — is about 2.5 mm. in the duckbill and 3.0 

 mm. in the anteater. The duckbill usually produces two eggs at a time, 

 the anteater only one. Apparently most, if not all, of the eggs come 

 from the larger left ovary. In many marsupials the eggs are much 

 larger than in placental mammals. 



In all mammals the embryonic Miillerian ducts open anteriorly 

 into the coelom near the ovaries and posteriorly into the cloaca. In the 

 later course of development each duct usually differentiates into three 

 regions: an anterior narrow Fallopian (or "uterine") tube which 

 serves to transmit the egg back into a more or less enlarged uterus, in 

 which the egg undergoes development for a period varying in different 

 mammals, and a posterior vaginal region (the right and left usually 

 joined), which serves to admit the male copulatory organ. 



The anterior end of each oviduct expands into a thin-walled funnel 

 whose mouth (ostium abdominale) usually has a fringed (fimbri- 

 ated) edge (Fig. 489). This funnel more or less completely enwraps 

 the ovary so that an ovum discharged by bursting of a follicle at the 

 surface of the ovary is caught within it. In many mammals the edge of 

 the ostium fuses more or less completely with a peritoneal fold which 

 surrounds the ovary so that escape of an egg into the coelom is hardly 

 possible. Otherwise, such escape sometimes happens and the misplaced 

 egg may become attached to the coelomic wall and begin to develop 

 there. Human cases of such extra-uterine pregnancy sometimes 

 occur. 



In placental mammals the funnel is lined by ciliated epithelium. 

 By beating of the cilia the minute egg is transported into the narrow 



