. Text-book of embryology. Embryology. IX MOLLUSCA 313 difficulty in tlie case of Faludina is that the complete series of earlier stages is not at all easy to obtain, since the earlier stages of development are passed through rapidly while the later stages of growth take much longer to accomplish. The chances are, therefore, that in any one womb nearly all the embryos will belong to the post-trochophoral or veliger stages; and Erlanger himself once told us that to find material for the adequate study of these earlier stages would require at least two months' search. The opportunities for comin
. Text-book of embryology. Embryology. IX MOLLUSCA 313 difficulty in tlie case of Faludina is that the complete series of earlier stages is not at all easy to obtain, since the earlier stages of development are passed through rapidly while the later stages of growth take much longer to accomplish. The chances are, therefore, that in any one womb nearly all the embryos will belong to the post-trochophoral or veliger stages; and Erlanger himself once told us that to find material for the adequate study of these earlier stages would require at least two months' search. The opportunities for coming to a decision in the matter are far fewer than in the case of an ordinary Gastropod, whose eggs are laid by thousands, and where every desired stage can be had in FiQ. 248.âHorizontal section througli the visceral hump of an older embryo ot Paludina â vivipara than that represented in Fig. 247, to show the formation of the kidneys and the heart. (After Drnmmond.) H, rudiment of heart; hep, liver; int, intestine ; , left kidney ; , left pericardial sac; , right kidney; , right pericardial sac; sept, evanescent septum between the pericardial sacs; st, stomach. Erlanger's account of the matter agrees in priHcipte with what is known of the development of Annelida with very small eggs, like Ewpomatus {Hydroides), where the mesodermal cells are budded out from the intestine. But of course the formation of a definite pouch is a far more primitive method of development than that so far described for any Annelid or Mollusc, and it is a somewhat strange thing that this mode of development should be found in Paludina, which cannot be described as a very primitive member of the class to which it belongs. There the matter niust rest, since Erlanger has been cut off by an untimely death, till some other embryologist has the patience to thoroughly investigate this difficult subject. When we reach the stage of the development of the pericardium. Please note tha
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