Sentence:  Stockard has shown that, not only do anesthetics arrest the development of the intra-embryonic blood vessels in the embryos of Fundulus, at an early ontogenetic stage, but in such a manner that no doubt can now exist that, under normal conditions, these vessels are formed "in situ" by a concrescence of independent and discontinuous <WSD>anlagen</WSD>, and that their endothelium is derived directly from intra-embryonic mesenchymal cells. 
Meaning of the word marked with <WSD> tags:  A primordium, the initial clustering of embryonic cells from which a body part develops.