STUDIES ON THE DEVELOPMENT OF THE HORSE. 



321 



During the fifteenth and sixteenth days marked progress is made by sheep 

 embryos. The 16-days 22-hours embryo figured by Bonnet has ten mesodermic 

 somites, a long deep neural groove, rudiments of the fore-, mid-, and hind-gut, and 

 two branchial arches ; the primitive streak is still distinct, the amnion is complete, 

 and there is a crescent-shaped allantois. 



In 1840 Hausmann figured a 19-days horse embryo with ten or eleven meso- 

 dermic somites. While generally agreeing with Bonnet's 16-days 22-hours sheep 

 embryo, Hausmann's 19-days horse embryo (text-fig. 19) has apparently no rudiment 

 of an allantois, and very little progress has been made in the development of either 

 the nervous system, gut, or branchial arches.* 



In a 17-days sheep embryo there are over twenty somites, the greater part of the 

 neural groove is converted into a canal, and there are indications of otic as well as of 



19. 



Text-fig. 19.- 

 Text-fig. 20.- 

 Text-fig. 21. ■ 



20. 



-Hausmann's figure of a 19-days 45-minutes horse embryo. 

 „ ,, 19-days 7-hours ,, 



,, ,, 19-days 26-hours ,, 



cerebral vesicles ; the fore- and hind-gut have assumed a definite form, and the 

 allantois has a width of 16 mm. ; further, there are branchial arches, a three-chambered 

 heart, two aortae, and a rudiment of the notochord (text-fig. 15). 



In a sheep embryo at the beginning of the nineteenth day there are more than 

 twenty mesodermic somites, and the development of the nervous system has advanced 

 so far that the three regions of the brain are easily recognised ; there are distinct optic 

 as well as otic vesicles, a three-chambered heart, a maxillary process from the first, 

 visceral arch, and a large allantois in connection with a well-developed hind-gut. 



The 21 -days horse embryo is in some respects intermediate between a 17-days 

 and a 19-days sheep embryo, but it obviously differs in shape, in the less-developed 

 condition of the hind-gut, and especially in the blastocyst, yolk-sac, allantois, and the 

 structure of the trophoblast. 



For some years it has been assumed that the rate of development in the horse 

 varies greatly during the first three weeks. This assumption is the result -of Bonnet 

 stating in 1891 that a 21 -days blastocyst varies from 13 mm. to 35 mm.f Now 



* Hausmann's 19-days horse embryo lends strong support to the view that Martin's so-called 21 -days embryo 

 represents the stage reached at the middle of the third week. 



f "Eier vom 21-Tage schwanken zwischen 1-3 cm.-35 cm. Lange," Bonnet, Grundriss des Entwickelungsgeschichle, 

 1891, p. 240. 



