ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 705 



lenticular thickening of the entoblast of the digestive tract, and its 

 cells wander or are thrust in, centrii^etally, to meet the centrifugal 

 growth of the ectoblastogenous mesoblast. Very soon, however, the 

 production of entoblastogenous mesoblast is observed to take place, not 

 merely at the periphery, but over the whole surface of the entoblast 

 of the digestive tract. Dr. Bonnet concludes that the cells of the 

 mesoblast are to be regarded as mesenchyraa in the sense of the 

 Hertwigs. 



The primitive thickening of the ectoblast grows caudally to form 

 the primitive streak, whilst the primitive concavity which is hollowed 

 out in it elongates caudally in a similar manner to form the primitive 

 groove. 



By the fourteenth day there is a cranial process of the primitive 

 streak, the first " rudiment of the ectoblastogenous chorda." 



The formation of the coelom in the sheep commences peripherally 

 from the disk in the outlying tract of mesoblast, and progresses 

 centrifugally, its proximal limit being formed by the very distinct 

 mesoblast-forming border of the entoblast of the digestive tract, now 

 underlying the growing disk. 



The author has found a canal piercing the cranial process of 

 the primitive streak, and placing in (temporary) communication 

 (1) the surface of the epiblastic tlaickening from which the neural 

 canal is later formed, and (2) the digestive cavity. This canal ho 

 identifies with Balfour's neurenteric canal. 



The first beginning of the blood-vessels was discovered in the 

 proximal region of the coelom, external to the embryo, and was seen 

 to arise contemporaneously in both layers of the mesoblast at this 

 point, developing centrifugally at a later period. As both layers of 

 mesoblast are, according to Dr. Bonnet, entoblastic, arising in the 

 first instance at the border of the entoblast of the digestive tract, 

 there is proof of the " indirectly entoblastic origin of the rudiments 

 of the blood-vessels." 



Dr. Bonnet promises a further paper on the further development 

 of the sheep's embryo. 



Development of the Generative Organs.* — 0. Cadiat has an 

 important memoir on the development of the generative organs in the 

 embryos of the sheep and of man. The results are as follows : — 



The internal always appear before the external generative struc- 

 tures. The cloacal cavity, which is formed early, commences to divide 

 in embryos of 8 mm. long into an intestinal and genito-urinary portion ; 

 but at the lower end of embryos of this age the two are still in com- 

 munication. When the embryo has attained to 1 cm. in length the 

 separation has advanced somewhat further back, but at the level of 

 the caudal extremity there is always a small common cloacal cavity. 

 In embryos a little older (12 mm.) the genital become partly sepa- 

 rated from tlic urinary passages ; it is not until much later (embryos 

 of 6-7 cm.) that the separation between the intestinal, genital, and 

 urinary tubes is complete. 



* Jonm. Anat. ft I'liysiol., xx. (1881) pp. 212-5D (4 pis.)- 



