456 ZOOLOGY FOR MEDICAL STUDENTS chap. 



from the outer world is much more complete, the young animal being 

 imprisoned within an egg shell, and in such cases the development 

 becomes modified — the larval characteristics tending to disappear while 

 features adaptive to an embryonic condition make their appearance. 



It is in the Tetrapoda that we find, correlated with their assumption 

 of a land existence, the most interesting modifications of the developmental 

 process. The Amphibia as & group have not succeeded in emancipating 

 themselves entirely from the ancestral aquatic environment. They as a 

 rule still require to pass through a fish-like larval stage in the water.^ 

 But the Reptiles, Birds and Mammals have succeeded in becoming 

 permanent denizens of the land. The developmental arrangements which 

 have played a main part in enabling them to do this may best be illus- 

 trated by' running over the main features in the development of a typical 

 Bird such as an ordinary fowl. It is easy to obtain eggs that have 

 been incubated for a definite number of hours and to examine their 

 contents by opening them in normal saline solution heated to about blood 

 temperature. 



The macrogamete or egg in the strict sense is the yellow or " yolk " 

 of everyday language. It is a gigantic spherical cell packed with reserve 

 food-material or yolk, its superficial layer condensed to form a thin 

 cuticle — the vitelline membrane. The living cytoplasm through the 

 greater part of the egg is sparse to vanishing point but at the upper or 

 apical pole there remains a little cap of cytoplasm — the germinal disc — 

 which shows up distinctly by its white colour against the deep yellow of 

 the rest of the egg. In the germinal disc lies the nucleus. 



When shed from the ovary the egg is sucked into the trumpet-like 

 coelomic end of the oviduct within which, if spermatozoa have been 

 received from the male, it is fertilized. The egg proceeds down the 

 oviduct, propelled by the contraction of its muscular walls. As it passes 

 onwards it becomes enwrapped in clear jelly-like secretion — the albumen 

 or " white '' (Fig. 191, alb) — secreted by the oviducal lining and taking 

 a characteristic shape — the pressure of the oviducal wall as it forces the 

 egg onward giving the mass of albumen a more or less pronounced 

 pointed or conical form, while on the other side of the egg the albumen 

 bulging into the relaxed portion of the oviduct takes on a rounded form. 

 The broad end of the hen's " egg " is then the end which was directed 



* A sketch of various interesting adaptive arrangements which have tended 

 to lessen the dependence of various species of Amphibia upon the presence of 

 water during early stages of their development will be found in Lectures on 

 Hereiity and Sex, by Bower, Graham Kerr, and Agar, or in the present 

 writer's Embryology. 



