DEVELOPEMENT OF REPTILIA. 



G33 



nection with the mid-pair, as shown in fig. 332, p. 504, at a. 

 The returning blood from the expanding lungs leads to the deve- 

 lopement of a distinct chamber in the auricle, which finally be- 

 comes the left auricle. Partitions in the bulbus arteriosus effect 

 a distinct communication of the pulmonary arteries with the 

 ventricle, and a division of what now becomes ' aorta ' into two 

 trunks. Of these one is appropriated to the left of the primitive 

 pair of middle arches ; the other becomes the trunk of the right 

 arch of that pair, and also of the anterior pair in course of change 

 into brachial and carotid arteries. The ' ductus arteriosi,' between 

 the anterior and middle arches (fig. 332, a), are usually absorbed 

 (as at d, fig. 334) : those between the posterior and middle arches 

 (d, fig. 335) are longer retained through the same course of change. 

 The trunk, which gives off the carotids either exclusively or in 

 common with the brachials, is posterior in Reptiles to the trunk 

 of the left aorta, and to that of the pulmonary artery. With the 

 developeinent of septa in the bulbus, there proceeds a like change 

 in the ventricle itself, but it does not reach the condition of a 

 complete ' septum ventriculorum ' until the crocodilian type of 

 Hamiatocrya is attained (figs. 339, 340). 



The substitution of kidneys for Wolffian bodies is preceded by 

 an enlargement of the latter, fig. 443, /, at their middle part, 

 with attenuation of their ends : the 

 true kidneys begin to be formed at 

 the upper medial part, and their uri- 

 niferous tubes are larger and more 

 convoluted. The genital organs 

 appear as a narrow white band upon 

 the ventral side of the Wolffian body. 



The developement of the brain 

 closely resembles that in the Fish 

 (pp. 604, 607), but it soon bends 

 down at a stronger angle with the 

 myelon. The cerebellar fold is first 

 distinguishable ; afterwards the de- 

 flected anterior part of the encepha- 

 lon becomes divided into mesen- 

 cephalon, cerebrum, and olfactory 

 lobes, and the cerebrum speedily 

 attains the superiority of size which 

 distinguishes the brain of the Reptile from that of the Fish. The 

 pineal gland shows a large proportional size in the embryo Turtle, 

 as does also the e thalamus ' or lower lobe of the mesencephalon, 



443 



Embryo of Lacerta viridis. 



