168 EMBRYOLOGY. 



rudiments of the notochord and of the pro to vertebrae appear in 

 the central part of the middle laj-er of the germ ; the two 

 outer portions of this same layer the ventral laminae (Seiten- 

 platten) split into an upper parietal (Hautmuslcelplatte) and 

 a lower visceral lamella (Darmfaserplatte} ; between the cleft 

 or split thus formed is the rudiment of the pleuro-peritoneal 

 cavity. (Figs. 180-182.) 



At the same time, the rudiment of the Wollfian duct ap- 

 pears on the upper surface of the middle layer of the germ, 

 where the rudiments of the protovertebrae abut on the ventral 

 lamina?. In sections through the blastoderm made during the 

 second day (36-48 hours), the protrusion of the primary optic 

 vesicles out of the anterior cerebral vesicles may be studied as 

 well as the intrusion of the secondary eye vesicle into the 

 primary (see fig. 185 6), which proceeds simultaneously with 

 the formation of the rudiment of the lens by the thickening 

 and subsequent separation by constriction of the intruded 

 part of the corneal layer. Similarly the auditory vesicle pre- 

 sents itself as a pit-like depression of the same layer ; this pit 

 gradually deepens whilst the margins rise up and grow until 

 they fuse into one another, so as to form the auditory vesicles. 

 We may further notice the extrusion of the visceral lamella in 

 the region of the neck, which forms the wall of the heart vesi- 

 cle. Sections made on the second and the commencement of 

 the third day serve for the study of the development of the 

 amnion, as a fold-like elevation of the corneal and parietal 

 layers, as well as that of the intestinal groove, and of the 

 fovea cardiaca ( Vorderdanii) by the closing in of the epithelial 

 glandular layer (see fig. 181). The extrusion of the two pri- 

 mary hepatic ducts out of the tube so formed, its partition 

 into a posterior oesophageal and an anterior tracheal-tube, and 

 the extrusion of the lungs from the latter must be followed at 

 later stages of incubation. 



