. Text-book de l'embryologie. L'embryologie. INVEETEBEATA 502 CHAP. ressemble à un astéroïde, jeunes et pour cette raison Johannes MuUer inclus la larve de ver » » entre l'astéroïde de larves. Au fur et à mesure du développement de la partie antérieure de la larve diminue en taille, et le bras se développer hors adultes. Le epineural semblent à première espaces ouverts comme les bords des rainures, qui répondent. Le perihaemal sont survenir exactement comme dans Ophiothrix. La seule anomalie frappante dispose de cette évolution sont la méthode de la gastrulation et de la formation de l'cœlome. En ce qui concerne le premier point, je

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. Text-book of embryology. Embryology. 502 INVEETEBEATA CHAP. looks like a young Asteroid, and for this reason Johannes MuUer included the " worm-like larva " amongst the Asteroid larvae. As development proceeds the anterior part of the larva diminishes in size, and the adult arms grow out. The epineural spaces appear at first as open grooves, the edges of which meet. The perihaemal spaces arise exactly as in Ophiothrix. The only strikingly abnormal features about this development are the method of the gastrulation and of the formation of the coelom. With regard to the first point, it is a remarkably interesting fact that if the eggs of Ophiothrix be removed from the ovary and artificially fertilized, instead of being allowed to be spawned natur- ally, they often exhibit the same kind of early develop- ment as Grave has described for Ophiura hrevispina. The mesenchyme begins to be cut off in the early stages of segmentation, so that the blastocoele, from its very beginning, is filled with it, and the blastula is in reality a "morula." Invagination is a solid inwandering of cells, and, when this solid mass becomes hollowed out, a tongue of cells persists which projects into the lumen of the archenteron. Such larvae never develop the anterior vacuolated crest. This interesting fact leads us to form some such con- clusion as this. The processes of growth and cytotaxis, which are the underlying causes of the outward and visible "signs" of formation of mesenchyme, in- vagination, etc., are the same over a wide range of species. The relations of these processes to each other in degree of intensity and times of maximal intensity, in a word the differential equations ivhich connect them, differ as we pass from species to species, and in consequence the same processes give rise to very different visible results in different cases. By interfering with the normal relations in any one species, in the case of Ophiothrix, by fertilizing the egg befor

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