916 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



The amphibia show an array of protective devices for young. The mar- 

 supial frogs are most interesting. In Gastrotheca (Nototrema) pygmaea, the 

 "maternal purse," formed by cutaneous folds, spreads over the dorsal area of 

 the trunk, and an elongated opening in the middorsal line permits passage 

 into the sac (fig. 371 B). In Gastrotheca marsupiata, the opening of the dorsal 

 brood pouch is located in the sacral area (fig. 37 IC). The brood pouch of 

 Gastrotheca ovijera is similar to that of G. marsupiata (Noble, '31, pp. 60, 

 510). In some forms, such as G. weinlandii, the skin of the back is covered 

 by calcareous dermal plates and in such species Noble says the young are 

 "enclosed within a veritable coat of mail!" Lastly, mention may be made of 

 the little Chilean frog, Rhinoderma darwinii. In this instance the male frog 

 carries the few eggs and young, through metamorphosis, in his vocal pouches 

 (fig. 370D). (See Noble, 31, pp. 71 and 507.) 



C. Special Adaptations of the Extra-embryonic Membranes for 

 Uterine Existence 



1. Implantation 



a. Definition 



Implantation is the process whereby the embryo becomes attached to a nu- 

 tritional substrate. The term is applied generally to those embryos which be- 

 come associated intimately with the uterine wall. This is the common usage of 

 the term. However, it is well to point out that the embryos of teleost and elas- 

 mobranch fishes as well as those of reptiles, birds and prototherian mammals 

 become attached to the yolk substrate of the egg. Moreover, this attachment 

 entails the elaboration of an extra-blastular or extra-embryonic tissue (i.e., the 

 periblast tissue) of a syncytial nature similar to that present where embryos 

 attach intimately to the uterine wall in the higher mammals. Most vertebrate 

 embryos thus rely upon a process of implantation for nutritional support. 



b. Types of Implantation 



When implantation occurs in such a way that the embryo remains within 

 the lumen of the uterus while the extra-embryonic membranes make a super- 

 ficial attachment to the uterine mucosa, it is called central or superficial im- 

 plantation. This type of implantation is found in all cases of implantation in 

 lower vertebrates. In the marsupial mammals it is present in Perameles and 

 Dasyurus, and among the eutherian mammals in the pig, cow, rabbit, sheep, 

 dog, cat, etc. In the mouse and rat the early blastocyst comes to lie between 

 the uterine epithelial folds in an antimesometrial position. These folds soon 

 enclose the blastocyst almost completely (fig. 374A-C). This type of im- 

 plantation is called ecceutric implantation and it borders upon the complete 

 interstitial variety. In still other mammals, such as the guinea pig, man, chim- 

 panzee, the embryo burrows into the uterine mucosa below the epithelium 



