FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 



915 



UTERINE LUMEN 



MESOMETRIUM 



9 9- 



UTERINE GLAND 



Fig. 374. Placentation in the mouse. (A) Blastocyst within fold of the uterine 

 mucosa. (B) Longitudinal section of uterine site of placentation showing mesometrial 

 and antimesometria! aspects. (C) Later stage of conditions shown in B. Observe that 

 placentation of the embryo is in the antimesometrial side of the uterus. The placenta 

 is probably of the hemochorial relationship at first becoming hemoendothelial later as 

 in the rabbit. (See Mossman, '37.) (A~C, redrawn from Snell. 1941, The Early Embry- 

 ology of the Mouse. Blakiston, Philadelphia.) 



4. Uncommon or Specialized Structures as Protective 



Mechanisms 

 Many structures other than the oviduct are used by various vertebrate 

 species to accommodate and protect the developing egg. In the teleost, 

 Heterandria formosa, the eggs are retained within the ovary (Scrimshaw, '44). 

 Although a typical, teleostean, oil droplet is present in the egg which measures 

 0.39 mm. in diameter, it is not utilized until late in development, and most 

 of the nourishment is afforded by a vascular sac which partly encloses the 

 embryo. In the teleost, Gambusia affinis, the egg also develops in the ovarian 

 follicle, but, in this case, most of the nourishment is derived from yolk which 

 is contained within the egg. In the sea horses. Hippocampus, and in the pipe- 

 fishes, Syngnathus, the eggs are transferred to a pouch, formed by folds of 

 skin located in the ventral body wall of the male. Here the embryos develop 

 (fig. 106). Many teleost fish are "mouth breeders," that is, they carry the eggs 

 for various periods in the buccal cavity. 



