The descent of man / Wilhelm Boelsche.

  • Bölsche, Wilhelm, 1861-1939.
Date:
1923
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The descent of man / Wilhelm Boelsche. Source: Wellcome Collection.

    49/100 (page 45)
    THE DESCENT OF MAN 45 external nipples but they already possess regular mammal glands; the milk dribbles into the mouth of the young through a basin-shaped and sieve-like perforated depression of the skin. Also the posterior end of the spoonbill shows during its entire life only that one orifice for urine, sexual discharges and excreta, it is termed a cloaka, and the whole animal group of the spoonbills is called the monotremata. By their place in the system these monotremata stand on a decidedly lower level than the marsupials. Neither of the spoonbills form a placenta. In this sense they would there¬ fore take a side position on the level of the more extreme marsupials. The land spoonbill even possesses a regular sack. But if one examines the contents of this sack of the female , at a certain period one finds something that absolutely sepa¬ rates the spoonbills from the whole tribe of the marsupials. Something so strange and unexpected in a mammal is seen that the first news of it was long looked upon as a zoological fairy-tale, until finally definite research work proved the facts beyond all doubt. We have seen that in the sack of the maternal marsupial there lies at first a young babe which is still so imperfect and deviating from the later shape, that it has actually been termed a ^4arva. But in the pouch of the land spoonbill there is at this time no young at all yet, but — an egg. An egg with a parchment like shell reminding one of the eggs of reptiles. With the aquatic spoonbill which has no pouch the eggs (in this case numbering two) are deposited in a mole-like subter¬ ranean structure. An egg of the land-spoonbill which is in ' my possession has the size of a rather large grape and looks in shape and color like a small yellow plum. Like a chicken from its shell the real young of the spoonbill emerges after a while from this egg within the warm maternal sack, and in doing so uses a special organ to break the shell, an early developed single egg-tooth which drops off im¬ mediately it has done its work; such special egg-teeth for breaking open the eggshell occur in their most perfect develop¬ ment among the reptiles. Not until the young marsupial has been completely hatched does its milk nourishment begin, which dripples from that nipple-less depression of the skin, — the essentially typical procedure of the mammal. A genuine