THK MALAYAN SPECIES OF Ols'YCHOPHOUA. 53 



cance of which must be discussed. It is the small somite 

 (PI. 6, fig. 10 a^ ce. som.) situated iu front of and above the 

 somite usually described as the first. It has developed a cavity 

 only ou oue side ; on the other side it consists of a mere group 

 of nuclei. This is all that can be said of its structure, and 

 nothing is definitely known of its origin. It seems that it 

 cannot have been separated from the first somite, which 

 remains undivided until a much later stage in the develop- 

 ment. If this be true, it follows that the small somite in 

 question must be an independent structure, produced from 

 the mesodermal bands at a late stage and disappearing early, 

 and, as such, must have a very short existence. To judge 

 from its position, its form and structure, and its late forma- 

 tion and early disappearance, it w^ould seem that we are justi- 

 fied in concluding that it represents a somite which has been 

 reduced to the merest vestige : so vestigial is it that it may 

 not even develop a cavity at all, but remain as a group of 

 nuclei lying in the undivided cytoplasm. It may, perhaps, be 

 permissible to conclude, since it is not possible to regard any 

 other metamerically arranged organs as corresponding to this 

 somite — unless the dorsal lobe of the brain, that is the archi- 

 cerebrum, be so considered — that the structure, the nature 

 and significance of which is here discussed, is a true cerebral 

 somite which up to the present has not been discovered. 



The Third Embryo. — The third embryo, the internal 

 structure of which will be considered, is the one shown in the 

 correspond iug figure on PI. 5, and is represented by four 

 drawings of sections, marked 11a, 11^, lie, and 11 tZ on 

 PI. 7, in which the first, second, third, and fourth somites 

 are respectively illustrated. 



On the ventral and dorsal surfaces the ectoderm forms a 

 thin layer, in which the tangentially compressed nuclei are 

 situated at some distance from one another, and, in the mid- 

 region, as well as towards the posterior end, is still in con- 

 tact with the endoderm (PI. 7, figs, lie and 11 d). 



Tlie rudiments of all the appendages have appeared as out- 

 growths of the dorsal moiety of the lateral thickeuing of 



