IV. Embryology. 



49- 



As to the development of the alimentary canal: the 

 primitive opening, the blastopore, usually closes, so that the hypo- 

 blastic tube is represented for a long time by a closed sac. Later, 

 the epiblast at each end of the animal invaginates to form, 

 respectively, the stomodaeum and the proctodaeum (Fig. 41). The 

 epiblastic invaginations and the hypoblastic tube, break into com- 

 munication at their points of contact, so that the complete alimentary 

 canal consists consequently, partly of the primitive hypoblastic tube, 

 partly of these epiblastic invaginations, and to these are added, in 

 many animals, elements from the mesoblast lying outside the canal, 

 and forming its connective tissue and the muscular layers. 



When the digestive tract contains mucli food-yolk, the young animal or 

 embryo is much distended : a yolk sac, an outgrowth of the alimentary canal 

 containing food-yolk, and smrounded by a con-esponding outgrowth of the body- 



Pig. 40. Embryo of the Dog-fish 

 with a yolk sac. — Orig. 



Fig^. 41. Diagram- 

 of a young embryo- 

 ■with yolk sac. 

 Longitudinal section. 

 Shewing also the. 

 formation of mouth 

 and rectum, d Pood 

 yolk, eh epi-, en 

 hypo-, me meso-blast, 

 m mouth. — Orig. 



wall, is then often formed. It is frequently much constricted, so that its cavity is 

 connected with the rest of the alimentary canal only by a nan-ow opening. Its 

 size is often relatively immense, so that the yoimg animal looks more like an 

 appendage of the yolk sac, than the yolk sac of the young animal. As 

 development advances, the sac gradually becomes smaller, and finally disappears 

 altogether. 



Most animals are oviparous, i.e., the egg is laid, surrounded 

 by the egg-membranes, whether fertilised or not. Sometimes even 

 after it is laid, it is still a single cell, but in other cases segmentation 

 has already taken place, or development may be even further 

 advanced. Other animals are viviparous, i.e., the ovum stays 

 so long in the body of the parent, that the egg-membranes or shell 

 are burst before the young organism leaves it. The distinction 

 between oviparous and viviparous animals goes no deeper than this, 

 and some forms are transitional between the two, for development 



E 



