ON THE REPRODUCTION AND DEVELOPMENT OF 

 ANIMALS. 



BY THE EEV. W. HOUGHTON, M.A., F.L.S. 



By the term development 13 to be understood the ■whole series of changes 

 that takes place in the Ufe-history of any animal, from the germ to the tim« 

 when it attains its adult form. Relatively to a eexual mode of reproduction, 

 ttnimala may be divided into three divisions, namely : — 



I. — Those which produce ova whose embryos contract a vascxdar connec- 

 tion with the uterus, and arrive at a perfect form within the body of the 

 parent. 



II. — Those which produce ova whose embryos do not contract a vascular 

 connection with the uterus, and which undergo their development, partial or 

 complete, within the body of the parent. 



IIL — Those which produce ova, which undergo their development after 

 deposition by the parent. 



The three-fold phenomena represented above are generally expressed by 

 the terms. Viviparous or Placental, Ovo-viviparous, and Viviparous, respec- 

 tively. 



In the first division are included all the Mammalia with the exception of 

 the Marsupials, whose embryo never contracts a vascular connection with the 

 uterus, from which it is expelled at an early period, and is then received into 

 the abdominal pouch. In the second division, the Ovo-viviparous, are included 

 the Marsupials, a few fishes as the viviparous blenny (Zoarcts viviparus), whose 

 young at birth are fully formed ; a few species amongst the Plagiostomooa 

 tribe of fishes, as some of the sharks and dog fish, several reptiles, as our 

 common viper, the slow worm, and viviparous lizard ; it is probable, however, 

 that in some of these cases laceration of the thin egg-membrane takes place, and 

 is occasioned by parturition, for an American boa constrictor has been known to 

 produce both young and eggs at the same tims. Amongst Molluscs several 

 bivalves are ovo-viviparous, as Cyclas, Kellia, Pisidium, Unio, Arwdonta, which 

 retain their fry within the mantle or the giUs, where they undergo either 

 complete or partial development. Amongst ovo-viviparous Univalves I may 

 mention Paludina vivipara. Helix rupeMvis, and Pupa umblicaia. The scorpion 

 produces its young alive, and in this curiously enough, the embryo is developed 

 in the ovum while stiU in the ovary. The scorpion family is aa sxoeptioa to 

 nearly all the other Aracknida, which are oviparoui. 



