VERTEBRATES. 3 



resulted in a great improvement of anatomical nomenclature. These efforts have been 

 ably forwarded by Professor Wilder in this country, with whom several of the terms 

 employed here, especially in the description of the anatomy of the brain, originated. 



Vertebrates are described in the ordinary prone attitude ; the upper surface is the 

 dorsal aspect ; the lower, ventral ; the sides, lateral ; the extreme front end is the 

 cephalic extremity; the tip of the tail, the caudal extremity. Their bodies are com- 

 posed of symmetrical halves, disposed right and left of a mesal sagittal plane ; i. e., a 

 vertical plane which joins the middle line of the dorsal and ventral surfaces from the 

 cephalic to the caudal extremity. Frontal planes pass from dorsal to ventral aspects 

 at right angles to sagittal planes. Horizontal planes cross from side to side at right 

 angles to both. Direction towards particular aspects of the body is indicated by the 

 adverbial termination 'ad;' thus, dorsad, cephalad, mesad. The symmetry of the 

 vertebrate body is disturbed by the intestinal organs, but it extends to the paired 

 limbs, anterior and posterior, which are generally present. Unpaired limbs (median 

 fins) are only present in aquatic forms. 



Certain of the systems of organs are disposed in segments, or metameres, one after 

 the other. It is not uncommon that contiguous metameric parts should be fused with 

 each other in such a way as to render it difficult or impossible to detect the segmen- 

 tation. In comparing such segmental or metameric pai"ts, they are said to be homo- 

 dynamous, while parts which are developed the same way in different animals are 

 homologous with each other, and structures which merely discharge the same functions 

 are called analogous. 



The intestinal organs show no trace of being disposed in metameres, unless the gill 

 clefts and homologous structures are thus interpreted ; and the same is true of the urinary 

 and reproductive organs, with the reservation that segmental structure is indicated in the 

 development of the former of these, and may be traced in the adults of lower vertebrates. 



With extremely rare exceptions, the sexes are separate in vertebrates ; except in 

 Tunicates no asexual reproduction such as is met with in many of the lower branches 

 occurs, and new individuals are consequently always developed from fertilized eggs. 

 The fertilization of the egg is effected by sperm-cells formed in the testes of male 

 individuals, while the eggs themselves are formed in the ovaries of females. 



The development of the egg takes place either entirely outside the body of the 

 mother, when the animal is said to be oviparous, or to a greater or less extent within 

 the body of the viviparous mother. 



Although in different groups we find the eggs as they leave the ovary to be widely 

 different in size, yet they are always single cells. In the craniate vertebrates, the 

 largest eggs are those of the birds, the smallest those of the higher mammals. In the 

 latter group the ovarian egg measures about -jj^y of an inch in diameter, the part of 

 the sperm-cell which fuses with it in the pi'ocess of fertilization hardly so much as 

 s-Ts^TT) so that it is from the conjugation of two quite microscopic elements that the 

 adult body results. In view of the facts of heredity, it is safe to say that the sim- 

 plicity of the structure of the reproductive elements is only apparent, and that, 

 although unicellular, they are still, in virtue of their molecular constitution, epitomes 

 of the structure of the adults. 



The sperm cells are always of small size, but the eggs may be very large, in virtue 

 of having within the egg-membrane a greater or less quantity of ' food-yolk ' in addi- 

 tion to the formative yolk, which is directly converted into the tissues of the embryo. 

 Further nutritive material, like the white of the bird's egg, may be furnished by the 



