THE OOLOGIST. 



179 



Cerelro Spinal Corrf and represents the 

 brain and spinal cord. We have so 

 far traced t e formation of an ol)lon{]f 

 sac al)ove which runs a hollow canal, 

 but another step has been also going on 

 which is somewhat more complex. 

 Th(; Blastoderm has split into two layers 

 thus giving us really two sacs, one with- 

 in the other and quite separate except 

 beneath the Cerelro Spiual Cord where 

 the Blastoderm remains undivided 

 through life. From the external sac, 

 the l)ody walls, the skeleton, the limbs, 

 the heart and blood are formed; from 

 the internal, the intestines and internal 

 organs. Four bud like pi'ocesses grow 

 out from the external sac These become 

 the four limbs. The internal sac grows 

 longer, much longer than the others 

 within which it coils, while from its 

 outgrowths, or buds, form the Liver 

 and Jjiings. The large end of the 

 Cerelro Spinal Canal increases in size 

 by thickening of its walls and the brain 

 takes definite shape. So far we have 

 no openings to or from the intestinal 

 canal, but at each end the walls of the 

 two sacs grow to each other and unite, 

 tne septum between them becomes 

 absorljed and the mouth and arms re- 

 main. Notice that neither of these 

 openings communicates with the space 

 between the sacs (that is between the 

 intestinesand the walls of the abdomen) 

 whieh in entirely closed. The eyes are 

 formed in a curious manner by an in- 

 growth from the skin meeting an out- 

 growth from the hair. 



By this time our «mbryo is decidedly 

 liirdlike in appearance although still 

 (Hcnpying but a small part of iis sludl 

 ;ind having the large unabsorbed yolk 

 sae attached to its abdomen. Fnnn 

 this point its history is more a matter 

 of "growth" than transformation ex- 

 cept in its developement of special or- 

 gans, a description of which would be 

 far too complicated for an outline such 

 as tliis. There lemains, however, a 

 structure which must be mentioned, 



and this is the membrane which en- 

 velops the embi-yo and wWwM occasion- 

 ally causes oologists so much annoyance 

 by clinging to the inside of the shell of 

 incubated eggs. Its structure is ex- 

 tremely complex and it mu^t suffice to 

 say that it is formed of three layers, 

 the outer being the Vitelline Membrane. 

 Beneath this is a delicate layer formed 

 by an outgrowth from the external sac, 

 which has budded out and grown en- 

 tirely around the chick. This is the 

 Amnion, its purpose being chietiy pro- 

 tective and hence more important in 

 mammals. The last or inmost layer is 

 a somewhat similar outgrowth from 

 the inner sac but its purpose is very 

 different, for the Allajitois serves the 

 unborn chick as a breathing apparatus, 

 absorbing oxygen and giving off waste 

 products through the porou** shell. 

 When the bird leaves the shell its lungs 

 of course become active, and the al- 

 lantois is thrown off with the rest of 

 the membrane. As mentioned before 

 these structures are of great importance 

 and of early formation, commencing 

 before the edges of the Blastoderm come 

 near uniting, although for simplicities 

 sake 1 have referred to them as out- 

 growths of the two "sacs". As an aid 

 in explaining the Amnion and AUantois 

 I would suggest their mental compar- 

 ison to the empty sleeves of a man's 

 coat and shirt respectively, without the 

 lower openings, and it can I think 

 readily be seen how they. may extend 

 and cover the entire body. 



THEODOUK \V. RlCH.\KI)S. 



Washington, L). C. 



THE WORLDS PAIR. 



BY Ol IK SPECIAL OOKRESl'ONDKN P. 



There has b<^en a great deal written 

 al)out the Columl)ian Exposition, and 

 tlu! newspapers an^ full of a general 

 description of the sights and i>rinciple 

 features. 3 But the naluralisls of our 



