These two kinds of argument support rather different conclusions. A vegetarian of the first sort has no grounds for objecting to the eating of animals—molluscs for example—too rudimentary in their development to feel pain. Nor could he object to meat-eating if the slaughter were completely painless and the raising of animals at least as comfortable as life in the wild. Nor could he object to the painless killing of wild animals. Such a vegetarian will, however, object to the drinking of milk, since the production of milk requires a painful separation between cow and calf. He will also object to the eating of eggs laid by hens which did not have scope for normal activity. (He will not, however, object to the eating of fertile eggs as such.) To that extent, he will be not only a vegetarian, but also a vegan, one who abstains not only from meat but also from animal products.
One might of course defend the consumption of animal products, while opposing the eating of meat, on the ground that killing a steer, say, produces more suffering than separating a cow from her calf. The argument seems to me a chancy one, but an intermediate kind of vegetarian on this kind of ground does seem possible.
In contrast, a vegetarian who has objections only to the killing of animals will object to all forms of meat, but he will not object to milk or eggs, so long as the eggs are not fertile. For such a vegetarian, a borderline case would be the consumption of animal products not, in the ordinary course of nature, produced by the animal; for instance the drinking of cattle blood as practised by the Masai. Of course one could be a vegetarian on both grounds, and object to anything either kind of vegetarian objects to.
(Philip E. Devine, "The Moral Basis of Vegetarianism," Philosophy 53 [October 1978]: 481-505, at 482)