ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 175 



wards. The lower layer, which goes to form the endoderm, does not 

 correspond to more than one-third of the blastula ; this imdergoes 

 invagination, the fluid of the cleavage cavity becomes absorbed, and 

 bilateral symmetry soon becomes well marked. 



In the " third period " the primitive segments, the nervous system, 

 and the notochord begin to be ajiparent ; the remnant of the blastopore 

 persists as an opening between the enteric cavity and the nerve-tube, 

 representing the typical neuro-enteric canal. Contemporaneously 

 with the development of the nerve-tube, the mesoderm developes the 

 primitive segments ; two lateral longitudinal folds arise in the dorsal 

 portion of the endoderm, and represent the rudiments of the mesoderm. 

 The cavities of the primitive segments are diverticula from the 

 archenteric cavity. 



After describing these points in detail, the author makes some 

 observations on the mechanics of the developmental process. All 

 those described are referable to foldings, solutions of continuity, 

 or outgrowths. What are the causes of the first ? Some are due 

 to contractions of the protoplasm, to active changes of form, while 

 others are referable to growth ; in the others we have to note as an 

 important factor differences in growth-energy. Active changes are 

 limited to short periods, and the formation of the dorsal groove is an 

 example ; with this the development of the mesodermal folds has a 

 close mechanical connection. Growth is more energetic in the 

 anterior regions. The prime cause of the development of the meso- 

 dermal folds would appear to be the greater superficial extension of 

 the endoderm in the dorsal region ; so, again, the formation of the 

 first primitive segment commences by a flattening of the anteriorly 

 placed endodermal cells ; the cells pushed back are folded transversely, 

 and so give rise to the first primitive segment. 



In the stage in which there are seven primitive segments there 

 appear in front of them two dorsal folds of the endoderm ; these 

 become more and more marked, and give rise to two blind sacs, which 

 are at first bilaterally symmetrical, and are placed at the anterior 

 end of the enteric canal. About this time the epithelial cells become 

 for the most part much more flattened ; and the dissolution of the 

 yolk- granules is almost completed. 



The fourth stage is the period of histological differentiation, 

 muscles become apparent, the notochord undergoes histological 

 differentiation, and fibrous cords appear in the medullary tube. At 

 the same time the larva alters greatly in form, becomes elongated and 

 compressed, and takes on generally a piscine character. The increase 

 in the number of primitive segments goes on but slowly, but what are 

 formed gradually fuse in the ventral median line. Each muscle-cell 

 has at first only a single fibril, and there is no indication of segmenta- 

 tion ; we may say, indeed, that a row of cells secretes a common fibril, 

 which is continuous throughout the length of the body. The 

 author cannot agree with Kowalevsky in thinking that there is any 

 special chordal sheath, and he does not see here any difference from 

 what obtains generally throughout the Vertebrata in the histological 

 differentiation of the notochord ; small vacuoles appear within its 



