the form of a shield, which becomes elongated opposite the 

 frontal projection; thus it appears doubly split. 9 



A special membrane covers the spinal cord. The cerebellum 

 in the embryo at that age projects considerably farther. The 

 canal between the cerebellum and the four-hillock structure 

 extends more than before; this corresponds with the posterior 

 part of the water passage (urethra) . The vesicle of the 

 four-hillock structure is strongly enlarged. The cavity, 

 corresponding to the third brain vesicle, grows insignifi- 

 cantly, but its bottom becomes elongated. The brain stalks 

 become more prominent than before, and become the brain stems. 

 The external membrane of the eye splits into two layers: a 

 hard membrane whose continuation is the cornea, and a vascular 

 membrane . 



Section 8 concludes the second period of development. 

 Here, Baer distinguished three types of processes: a continuing 

 separation of the embryo and its envelopment by membranes; the 

 separation of the vegetative from the animal part; the turning 

 of the embryo to the left and the displacement of the blood 

 supply to the left side. The final process represents the 

 progressive process of internal differentiations, which are 

 entirely characteristic of the vertebrate type. 



The separation (unlacing) of the embryo from the blastoderm 

 outside the embryo takes place by the appearance of folds — 

 anterior, caudal and lateral, all representing parts of one 

 common fold, which is transformed at the final end into the 

 umbilicus . 



The turning of the embryo to the left side, and the 

 corresponding displacement of the vessels, Baer compared to 

 the particularities of development in molluscs. Observing the 

 similarity, Baer made a reservation: "It is impossible, 

 however, to say that the chick embryo is now present at the 

 level of development of molluscs. Against that, the presence 

 of the vertebral column, the spinal cord, and the brain are a 

 definite proof" (I 8 c, p. 142 (89)). 



9. This fits the description of the development of the face 

 skeleton of the chicken embryo given by Tredern and his 

 drawings (see pp. 125 - 126 and drawing 18) . 



328 



