80 SUMMARY OF CURRENT RESEARCHES RELATING TO 



140 pages, and are accompanied by drawings, partly due to Herr W. 

 Winter, which exhibit unusual skill and care. 



I. Blastoderm and germinal layers. — (1) The egg-memhranes round the 

 blastosphere are first described. The firm and tense chorion, the delicate 

 blastoderm membrane, and a third structure covering part of the external 

 surface of the chorion are noted in detail. The contents exhibited nume- 

 rous oil-globules, yolk-pyramids, and small white yolk-elements. The 

 central body found in the yolk is regarded as a residue of the undivided 

 yolk. The blastoderm extends all over the yolk and is not, as Eatke 

 supposed, confined to one region. (2) Germinal layers, &c. In stage A 

 the embryo is an aj)proximately spherical closed sac of a single layer of 

 cells except in one region. The centre of gravity is in the hemisphere 

 opposite the ventral plate, and thus the latter lies uppermost. The anterior 

 region of the ventral plate exhibits the head-lappets with cells arranged in 

 definitely concentric curves. The central portion exhibits the beginning of 

 the ocular invagination, and the crystalliue-cone-cells were also detected. 

 Posteriorly the thoracico-abdominal rudiments were seen, and behind these 

 a fifth largo cell- plate — the endoderm disc. The mesoderm first aj)pears 

 at the anterior region of the endoderm plate. Stage B is characterized by 

 the semicircular gastral groove on the endoderm plate, and this in stage C 

 becomes annular. The mouth of the gastrula is primitively a circular, and 

 afterwards an oval aperture with the narrower portion situated anteriorly. 

 The formation of the primitive mouth varies considerably, and in this 

 something more than intensity of cell growth and consequent pressures 

 must be regarded as influential. The mesoderm developes at the passage 

 of outer into inner layer at the anterior margin of the primitive mouth. 

 In stage D the ventral plate is heart-shaped, and the primitive mouth 

 becomes closed. The embryonic rudiment becomes considerably reduced 

 both in length and breadth. The head-lobes and eye-rudiments have 

 approached one another in the middle, and the thoracico-abdominal plates 

 are united medianly. Reichenbach lays considerable emphasis on the 

 differentiation of the mesoderm into primary and secondary. He describes 

 how the endoderm cells devour the yolk-elements. (3) General, The third 

 chapter contains a useful comparative survey of the relative literature. 

 Eeichenbach also calls special attention to the regular curves in which the 

 cells of the embryo are disposed, as the beautiful plate s so well illustrate. 



II. Body-form and systems of Organs. — After describing at length the 

 features of the embryo when the three nauplius appendages are distinctly 

 apparent, the author gives a detailed account of the next five stages — 

 (^) with developed masticatory appendages, (Ji) with developed walking 

 limbs, (/) with abdominal appendages, (k) with well-developed eye-pigment, 

 and lastly (I) the liberated embryo. Of these modifications it is hardly 

 possible to give any brief account. 



Derivatives of the ectoderm. — (1) The history of the external skin is first 

 described. Particular attention is directed to the internal prolongations 

 which serve, along with certain mesoderm elements, for the insertion of 

 muscles, for sinews, supporting beams, &c. In the section of the carapace 

 the interstices contain large wandering cells with yolk-like contents. 

 (2) The nervous system. In the four head-segments of the uaujilius, four 

 pairs of ganglionic pads appear as ectodermal thickenings ; of these the 

 two last are separated by a shallow median furrow, which extends to the 

 budding zone of the thoracico-abdominal rudiment. The first pair of 

 ganglionic rudiments belong to the eye, the second and third form the 

 brain, the fourth the ventral cord. The development is thence described 

 at great length. From the very earliest stages the large ganglionic cells 



